I know some guys may not consider 5 acres to be all that much to mow, but it is lot of land to mow, especially for a homeonwer who doesn't do it day in and day out, and the larger machine will be more comfortable once you're on it out on your property. The larger machine isn't overkill IMO. It is just a bit more overall than the others and that added weight/mass goes towards longevity of the machine's lifespan generally. It makes for a better ride, and better long term durability. But..... if you have wet spots that tend to stay wet or at least damp, the lighter the machine, the better. If your soil drains well and is basically dry outside of rainy times, the 3100 is one heck of a nice machine, and the Vanguard BB engines are excellent.
BTW- I run a Z950A.