Hi:
One thing you haven't mentioned is the type of Soil you mow on. If it's very soft you might want to stay away from the heavier 3000 series.
I have a 2305 (62"MMM) and a 3720 cab with a 72" MMM (Just traded in a 3320) - I use them on different properties. I can't speak for the 2320 or 2520, but you can mow much faster ground speed with a 2305 than you can with a 3320 (or any 3000 series). The reason is stability. Yes, you get an extra foot with the 72" mower but I'll bet you can go more than that much faster with a 2305. I just mowed 5-6 acres (That I used to mow with the 2305) for the first time on the brand new 3720 (Have mowed it with the 3320 also) and if I tried to mow anything above maybe 6mph (I can get to almost 7 at times) I'd bounce off the tractor of flip over Some areas I need to go below 5mph. With the 2305 I could (can) mow full open in high (About 9.1mph) and live to tell about it. Don't get me wrong, I love the cab, but having both I can tell you the 2305 can outspeed the 3x20 series just due to stability (Again, if I consider the 16% wider mowing area it maybe closer). Also, if you have any slopes the 2305 'feels' much more stable. You'll reach the 'pucker factor' real quick on a 3000 series with even a slight side slope.
If, you were going to put a big pull behind (84"+) then I'd reverse my judgement and say the 3x20 series would beat the 2305 easily.
Remember, this is 'finish' mowing. You will not handle the big implements on a 2305 that you can handle on the 3020 series but it'll be easier to trailer, maneuver, etc. I have both so I'm speaking from the mindset of loving them both.
I cannot speak to compare the 2320 and 2520 to the 2305. I'm not sure if the bigger frames would take away from stability but they might. If you need ground clearance the 2305 may not be your ride as it's very low.
If finish mowing is mostly what you'll be doing, the 2305 might be a great fit. It's also the cheapest of the bunch and accessories aren't bad.
A snowblower on a 2305 will run you less than 3k (2k for the blower and a maybe 600-700 for the mount) - I have one. On the 3000 series it's about 5k (YES, 5K). High 3's for the blower, and over a grand for the mounts. (I have one on order too).
Basically, EVERYTHING is less for the 2000 series than the 3000 series. The price difference in the tractors is the smallest difference.
Hey, if $$ is no object buy a 4720 cab and mow with it..

I have 'overbought' in both my cases.
Just my .02. (Well, I guess maybe .02+.02+.02 due to my blabbyness)....