Get the 12' the extra 3' makes a big difference in a hurry, you are getting a free pass every 3 passes over the 9'  and 95hp is more than enough we've ran up to a 15' haybine with 80-85hp 2wd 2840 and 4wd 2950 at our elevation it is probably putting out 70 or less-- works fine.  Ran up to a 12' disc as well.  When we had a 9' disc we ran it with a 2wd JD1650, that is about a 5Klb tractor and 65hp turbo HP worked great.  I've moved the 15' haybine around the yard with it, doesn't move the tractor as much as you'd think.  Also center pivot makes things go a lot faster.