Whitfield was bought out a few years ago by Lennox, so parts shouldn't be a problem. I have one of theirs, a PF-20 that has been working well for 2 years now. The Harmon's and the Quadrafires are very nice, as well as those made in Canada by Enviro Fire. I would say all 3 of these makes are built a little better than the Whitfield. Some of the cast iron models are much nicer. I did quite a bit of research before I bought mine, but I also got a pretty good deal on it at just under $1700 delivered, which ultimately made up our mind. I installed it in place of a freestanding wood stove.
I don't know how familliar you are with pellets, but the combustion process is powered by a blower on the flue outlet pipe. It places a vacume on the firebox which forces combustion air to be drawn in thru the burn pot under the pellets, from an air duct. Any leak in door or case seal will draw in air instead of letting smoke or gas out into the room.
One thing I wish I would have looked closer at was the combustion air inlet. You want one that is hard plummed all the way into the burn pot. IE, the only source for the combustion air is via a single pipe so you can draw all the combustion air from outside the house by connecting it to an external air duct. The Whitfield I bought has a small pipe for combustion air, but it is only thru the rear skin of the outer case, which is full of louvers and about as airtight as a birdcage. The actual combustion duct on the fire box draws it's air from the inside of the outer case, or basically from the room it is setting in.
Pellet stoves burn so clean by pumping a LOT of air thru the stove. I would say far more than a woodstove does. If the combustion air is drawn from the house, it is pumped outside via the flue pipe. This room air has to be made up from somewhere else, like any leak around a window or door seal with cold outside air. Our first season of running it was pretty chilly in the living room. The stove was pumping out heat like mad, but the outer edges of the room were always cold, way colder than they had been with the wood stove. I had to do some internal welding and ducting on this new stove to get the combustion source into a single duct that I could plumb to the outside. Once I could draw the combustion air from outside the house, it was much more comfortable inside.
I did not get the opportunity to take the sides off of the whitfield when shopping. The only one I found local, was a working demo and the store heater. The gentelman didn't sell Whitfield anymore. I found the one I bought on line from a supplier in Idaho. If I would have had the chance to look inside, this lack of ductwork would have been readilly apparent and taken it off my list. The rest of the stove is built pretty well, so the little bit of the pipe I saw comming out the back lead me to assume(as it was intended to) that it was hard plummed the the burn pot as in other stoves I have seen. Caviat Emptor... IT looks nice(wife liked this one's look) and after a little work by me, it is performing as well as any pellet would in our home, and the price was right. But I would NOT buy another Whitfield! My next vote based on performance VS construction VS Price was the small envirofire that had about the same 12,000 BTU output on low.
Another thing to look for is heat exchanger size. there is a second blower that pumps room air thru the heat exchanger tubes and back into the room. The size(surface area) of the heatexchanger is directly proportional to the efficiency More tubes are better. Most pellet stove heat exchanger efficiencies are in the area of 75-80% I found when I was doing my research, that some manufacturers advertised their stoves output by the ammount of fuel it burnt(in BTU) as this is a larger number and makes the stove look more impressive. Some listed theirs the more honest way by the actual heat output into the space. But since they all give burn rates in pounds per hour, this is easy enough to check out. Average grade wood pellets contain around 8500 BTU per pound. So a stove like mine that burns 1.8LB(around 15,300 BTU) on low only puts about 12,087 BTU into the room at the 79% advertised heatex efficiency. So the claimed 12K BTU on low was accurate. Usually some or all the parts to this equation are available in the stove info brochures, pounds burnt per hour, claimed heat output, or heatex efficiency, to see just how honestly the manufacturer is selling their product.
The above information is also handy in calculating the size stove you need. Most all the stoves I have looked at offer low, medium and high heat output levels. And I think with only one exception, all were reccomending NOT to run them on the highest setting for extended periods. They are also less efficient on the higher settings(more heat goes up the chimney) So you should size the stove to meet your needs on the lowest setting and the intermediate and high settings used only intermittently to take the edge off a cold room. Most all the stoves I have seen offer a thermostat option that turns the stove on or off for heat control. Some, like the Envirofire can also shift from high to low under thermostat control to maintain the temp in the room, I wish I had this feature, mine only does on or off. Since I don't run the stove wheni am not here, I don't even use the on or off feature.
Most all the stoves I have looked at are top fed, IE: The auger lifts the pellets up from the botom of the hopper to where they fall down a chute into the burn pot. I believe Harmon marketed a bottom fed version where the pellets are pushed into the bottom of the burnpot, which makes for a very steady and quiet feed method. I don't know if they make these anymore, but it didn't look especially safe to me, as if the power went out, and without forced airflow control, the potential existed for the fire to burn back along the pellets in the auger and ignite the pellets stored in the hopper. This is virtually impossible in the top fed design, which most all pellet stoves employ.
Another factor is noise. Mine sets in my living room along with the TV. It has 2 blowers and an auger motor, so it makes noise. It is not especially annoying, but it does add to the background noise level. One of the quietest stoves I looked at was the Envirofire Mini. It actually has insulation on the insides of the cabinet panels. Not for heat, but to dampen the blower noises and it was by far the quietest I have ever looked at(this one initially had my vote, but the wife didn't like the way it looked

. A insert model should be quieter, but look for insulation on the trim panels that will fill in the open space around the unit once installed in your existing fireplace.
Another issue may be necessary heat. My main source of heat was the woodstove that the pellet replaced. Pellet stoves require power to operate so I wound up building a small generator set for my home to keep the lights and now heat on. They don't take much power, mine uses about 400Watts in normal operation and about 700W when lighting. This was a little too much to feed long term with a inverter, so I went with a generator as this had other advantages.
Good Luck.