I am still playing my 40d fortress. It is over 200 dwarves, full size map (16x16) and FPS is pretty steady at 12. When there's ambushes on the map it drops to 9. It's positively fast compared to what 31 puts me through for much smaller maps. I have a nicely varied terrain in all that space - the fort is just an area at the center of the map, the rest of it is what I think of as my own little kingdom - the hinterland, the countryside nearby. Means I get to actually set things up like expeditions into the woods or nearby mountains to do things like hunt or clear-cut for some wood or strip mine some mineral veins before then having the expedition return to the city. Can't do that in 31 because you just can't take such a large area. As for underground stuff, my map has an underground river that is filled to the brim with cave crocodiles (breeding on their own for ages) and a chasm that was completely covered over by the mountain (not open to the sky) which is full of giant bats and giant cave swallows (I was lucky and had a breeding pair of the swallows get genned in there - normally I only see one at a time). That gives me plenty of stuff to do underground.
My military _is_ overpowered but all things considered I prefer that to the utter idiocy of the 31 military. What particularly made me snap about 31 was when I moved a dwarf who had all the ideal equipment for his skills from one squad to another, whereupon he dropped everything and went around picking up half-assed crap, because the way equipment works, it gets assigned to and owned by squads, not individual dwarves. It's also why woodcutters and miners can't get activated into the military and then deactivated and keep using their axe or pick for the various tasks the whole way through.
I admire Toady's creation as much as anyone on these boards, but that is just a totally fucking moronic way to implement it, and I'm not putting any more time into the 31 military until that gets fixed.
Also, for all the health care changes - I just DON'T CARE. A hit point system, and having them lie in bed for a couple years and maybe recover, is good enough. Surgery and bone-setting and crutches and splints and all the rest of it is just irrelevant to me.
Actually the one 31 game that I am playing is one where I found a "real" island - surrounded on all sides by water, all within the same tile (it's 6x6 embark I think) - so no immigrants. I'm playing it as shipwrecked dwarves, they've built a wooden stockade in the center of the (jungled, gorilla-inhabited) island (there's an aquifer, so digging down very far isn't easy) and I'm just letting things go and see the years pass. But if I want to play a real game, with sieges and cities and so on, 40d is the way to go.
(Actually, once I finally reach a stopping point with my earlier mentioned fortress, I have another 40d embark site I really want to try - a lonely ridge in the middle of a perfectly flat Haunted desert, and a dwarf civ that has lost all its cities but the king is still alive and reasonably young, and the goblin civ that did them in still has its demon leader. I have visions of turning that mountain into a ballista-bristling fortress of doom where the dwarves make their final stand, the king returns, and a final cataclysmic battle against the demon lord occurs. I generated a lot of 31 worlds to try to find a similar setup but no dice.)