I have a BX, and my B2601 will be here on Friday (fingers crossed).  I have around an acre, I mow maybe 3 acres because I do some roadside and local community land.  My property is flat, I have a few trees and various garden tasks, I pull the boat with the tractor occasionally.  I'm upgrading because I was getting a new tractor (my BX was that gateway tractor that I bought cheap, now I've proven I need a tractor, I'm getting a new one).  And if I'm buying a new tractor, the B is only slightly larger and a lot more capable.  It'll do everything the BX does, but more.  
I wouldn't consider a BX for a 4-5 acre property.  One thing to thing about beyond those already mentioned (ground clearance, lift capacity, position control, 3ph capacity) is speed.  The 3 range transmission makes a big difference, and 7 acres is enough that going 50% faster whereever you're going matters.  On property that size you may have a bit of mud too, the B tires are a lot larger = more traction.  It's easy to squash the BX front tires with a bit of load on the FEL.