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.