The 4100 has 19 HP Net (with 17 at the PTO). The B7500 has 21 GROSS HP (16 HP at the PTO (HST), 17 HP if gear tranny). Figure the HP losses when the gross HP (measured without alternator, cooling fans and other required accessories for real world usage...some of our mechanical friends can expand on this) converts to Net, one can expect a 1-3 HP drop. Net HP is a more usable figure for what you buy and use.
So, the two tractors are virtually identical when comparing power. Of course, one must look at gearing, vehicle weight, etc.
Which one is better? Your call...
But it sure seems both are capable tractors. The 4100 series cannot use a backhoe, if you're thinking about one. The B7500 can, but it'll be a small unit for the limited HP.
The B7500 was the machine that put me on a tractor search. The BX2200 was my first choice (solely due to lower C of G), since my usage is mowing and snow plowing.
Bought a used JD670 (almost identical to the JD 4100, spec-wise) for about 4500 bucks less then a new BX2200. The 670 is quite capable for my use.
There ya go, friend...another opinion from a friend on TBN!!