170 hours is very, very light usage on either of these. As you will see, horsepower isn't the issue between the two. Both will have 22/23 rated hp, with the BX having less at the PTO than the B model.
The B is simply a much larger tractor. It's bucket is larger, its tires are larger. The B has approximately twice the ground clearance and should be priced at significantly more dollars. $6000 plus or minus on the
BX2200, and $8500 plus or minus on the
B7510. (Those are low figures. A spotless machine could fetch up to $1500 more on the BX and $2000 more on the B, depending on attachments included. But that is all. Paying more might mean you'd be better off looking elsewhere or looking new with warranty and zero percent financing.
Look for a well cared for, almost pampered machines. Get a history of fluid/filter changes. Look for damage, dents, fluid leaks, etc. Only you can decide whether a BX or a B best suites your needs. This is based on turning radius requirements of your property and your planned use.
Finally, be very, very careful if you are responding to Craigslist or similar. If the price is too low, (from what I stated above) it is absolutely, most likely, a scam. Never give anyone a dime without laying your own eyes on the machine. The old "it is in South Carolina and will be shipped to you" trick is a total scam.