Having just been through this, but without a cheap 3100 complicating things, I see both sides of the issue. $500 buys a lot of toys to go with the mower, or a lot of beer to drink while mowing. Also, it's your budget, and only you know what will or won't break the bank. However, if a 3100 with a 50" deck had been available to me when I bought, there's no doubt which mower would be in my shed now. It's none of my business, but if you are planning to finance your new purchase Cub may still have 24 mos. 0% on leftover old year models. It isn't listed on CC's website, but my dealer knew about this incentive. At that rate, the 3100 would only be about an extra $20 per month. Truthfully, you don't need the 3100--the 2500 will do everything you want to do with your mower, and it will (should) continue to do everything reliably for the next 10-15 yrs with routine PM. However, for $500 I'd upgrade to the 3100. The 3100 has a 9 ga. frame versus 11 ga. on the 2500, plus it has a heavier transmission, a BDU-21 vs. a BDU-10, and a drive shaft running the deck as well as one running the trans. Not to mention the joy of only having to use one finger to guide the mower around the yard.
Basically, the 2500 is all the machine you need, and it is one fine, HD lawn mower, but the 3100 is pretty much like a baby sub-CUT. As I said before, you can buy either one and rest easy knowing that the new mower you bought will last a very long time and still be worth a good price when you are ready to upgrade. One final thought on the issue--when you get ready to upgrade someday, the 3100 should easily be worth at least $500 more than the 2500, so you should have no trouble recovering the additional investment in a 3100, and you may get even more than that.