It really does depend, but consider this... a ZTR is purpose designed for mowing grass. I have a 72" Exmark ZTR that is rated to mow 6 acres per hour. That would be on flat, even ground where you could go full speed - as a couple folks have mentioned it can get a bit bumpy on a ZTR at speed. If you haven't cleared and leveled your ground you won't be able to go anywhere near that fast. I've leveled most of my grounds and mow about 6 acres in around 3 hours.
The best way to mow fast is to get one of the larger mowers, like this: <http://www.toro.com/en-us/Golf/Mowers/Rough/Pages/Model.aspx?pid=Groundsmaster-4100-D> The Toro 4100 would probably be my first choice if money were no object, but these things get really expensive new. You can sometimes find them used at a surprisingly reasonable cost, but unless you can handle maintenance yourself operating costs are still pretty high. However, they're really fun, fast, deliver an awesome cut and can handle surprisingly rough terrain. The diesel versions are Kubota powered, which is true for a lot of commercial turf equipment.
It comes down to money, really. If you already have a tractor, that's the most cost effective way to go because despite a good mowing attachment's cost you still have only one piece of equipment to maintain. Don't rule out getting a lawn / garden tractor with a large deck - those can work pretty well and when you start looking at the cost of a deck for a tractor you're probably in the same neighborhood. Attachment manufacturers don't have the economy of scale in manufacturing that the residential mowers do which is why a specialized mower deck costs so much more than a roughly equivalent residential deck. Mower decks are somewhat different in construction as far as gauge of steel and such but really all any of them are is a steel frame with a blade held in a spindle, turned by a belt (except commercial hydraulic mowers). Not rocket science and when you consider cost, the residential stuff is cheaper even though it won't last as long.
Anyway, the big factors are speed and size. The faster you can go and the bigger the deck, the less time it takes. If you can go fast, get a specialized mower with a really high blade tip speed and you'll get it done in the least time. If you have to go slower, get the biggest deck you can. Cutting grass is just a blade whacking it off and unless you're really diligent about keeping your blades sharp you're just "bludgeoning" the grass (think weed eater with a really big, metal line) so brush hog vs. finish mower quality of cut is often similar.
Eventually you'll try all sorts of things and it'll come down to personal preference. Don't forget that most equipment dealers (especially the landscape suppliers) have demo models so you can and should be able to try something out to see if you like it. Lots of people get a ZTR and they scare the !@#$ out of themselves, because on a hill they can get quite squirrelly. Don't fork out that kind of cash without trying first!
