I was looking at the 1528 (yes you are asking about the 31, 28 and 31 use the same frame) before I got my 1533. After sitting on the 28 and the 33, I was more at home with the 33. I think by the time looking at the difference in price it was less then $2000 to get the bigger frame tractor. Now I don't know the difference in cost between the 31 and 33... I would think MAYBE the 31 could be close to the same if not more because (correct me if I am wrong), the 31 has a hydro trans with a 33hp engine and my 33 has a 8x8 Synchro trans and the same 33hp engine.
Some other things to think about when looking at the 31 (this is from the book)...
1531... In Hydro trans you have about 1 1/2 HP LESS at the PTO then if you got a geared trans. <<< again from the book.
1531... You have LESS Hydraulic flow by about 1.5 gal per min compared to the 1533
1531... is allot lighter tractor then the 1533 by 600+ lbs... Will you have the weight needed to put the 33 hp to the ground?
1531... 3pt will lift about 2,425lbs, the 33 will lift about 2,800lbs
1531... If you plan on getting a FEL you have to go with the SMALLER 1520, If you went with the 33 you can get the 1525 FEL, 1525 can lift 940lbs more and can go 12" higher then the 1520 FEL.
I know what I have told you ISN'T what you asked, but I feel I had to let you know. Again maybe the 1533 is to big for you needs.