If I'm not mistaken, I thought the new stuff they use (looks like molasses) doesn't rot the rims. The old stuff (Calcium chloride and water and other stuff) will rust the rim if exposed to air. That's why when using Calcium chloride you want to make sure they get filled completely over the rims.
I'd fill them and I'd also go for the 61" bucket.
'Oversize Lawn Tractor' - "Micro Compact'
(sniff sniff)..
Only kidding.
While the 2305 is physically small, having both a 3720 cab (had a 3320 open) and a 2305 I must say the 2305 is a 'tiny' workhorse. The work you can get out of it for it's size is incredible. It also has respectible PTO power. The hydraulics are slow, but it's got a power to weight ratio higher than any of the Compacts when it's at it's 'working weight'. Sure my 3720 is slick but I'm running a little shy of 6000lbs when I mow my lawn it's a heavy monster (filled tires and I take the bucket off but not the loader frame)..
Also mowing you really can't beat the 2305. I've said it before and I'll say it again. I really feel I can finish mow faster with the 2305 than I could with either my 3320 or the 3720 with the 72"MMM - Reason is stability and speed. I mow full out in high on the 2305 (About 9mph) and if I occasionally get up to 7 on my 3720 I'm about to bounce over on my roof). The extra 10" of width doesn't make up for the speed I get on the 2305. I am now running an 84" RFM on the 3720 and think I can now beat the 2305 due to a lot of extra width. (I can run the RFM at 4-7mph - Probably around 5.5-6 most of the time).
Anyway, just wanted to defend the 2305's honor...
One other item that's not obvious, the deck height adjustment on the 72"MMM on the 2520 I believe is that like the 3000 series, a non-marked wing nut with a long lead screw. This is not nearly as nice as the quick adjustment knob on the 2305/2320. You can change heights on the 2305/2320 while mowing if you are careful. You need to stop, get off, raise the deck and 'guess' (or count the number of turns of the turn nut) to adjust that on the others.
One other thing I remember hearing is that if you get a BH (Everyone needs a BH
I have a BH6000 on my 2305) I think you have to remove the MMM mounts (not just the deck) to mount the BH on the 2520 (not the 2320). I might be wrong on this but I believe I remember reading this here.