Staying level 31 doesn't make much sense IMO, because as has been said, you are quickly back at 20-25. If your build is incomplete before 31 it might make SOME sense...
I've also heard some people say heirloomed items limit them to a certain build, because they don't want to not use it. This doesn't affect me personally, though, and I have loomed three different weapons (1h, 2h and pole). I usually retire because after the grind from 30-31, I am fed up with my char and want to try another build.
I think the broad 1h axe would be a good weapon to heirloom, because it will do 38 cut dmg, which means 25 dmg avg with PS5, so you can 2-hit those str15 armor 40 guys quite often.