I've used my pressure cooker for stock recently more than the slow cooker and I do find I get much firmer "gel". Also, I don't find I need to use vinegar or lemon juice - the bones soften so much that I'm sure lots of minerals etc are leaching out already.
It is ideal for stock from one or two chicken carcasses, and for smaller beef or lamb bones. I give chicken about 40 minutes; haven't re-used the bones yet but will do next time. I doubt if the 2cnd stock will have much gelatine left but will certainly have mineral content. Beef and lamb I give longer, about an hour.
BIG beef bones - I have some in the freezer - I am going to cook in the slow cooker (they don't fit in the pressure cooker at all well), and if I can split the bones into smaller pieces after they have been in the slow cooker,do the 2cnd stock attempt in the pressure cooker.
I don't put ANY veg with the bones, just a few black pepper corns and bay leaves. The stock is delicious (slightly salted) as a drink, and awesome for soups and casseroles.