As you step up in size in the Kubota line of tractors, say the BX series up to the largest M series, what you are doing is going from something that is more like a large garden tractor (BX series) up to a pretty serious farm tractor (M series). The B is going to be lighter and better for finish mowing your lawn. Most of the time, it is just going to feel handier. If it wasn't for mowing the 40 acres, I'd say go with the B. The power may be about the same, but I would say the L would be better for brush hogging the 40 acres, just due to the size of the tires and the weight. If it is really smooth, the B might work just as well. The L is also going to be better for serious work such as moving dirt and gravel logs etc.
Take a hard look at both of them again, and imagine the tasks you will be doing and the number of hours performing each task, and that may help you decide. I bought a Bx about a year ago, mainly as a lawn mower, but I have found that half of my time has been FEL work, and I only own about 3 acres. I'd say get an L and buy another mower for finish mowing around your house when the current one dies. If you don't want to buy a seperate mower in the future, get the B.