turbos will ALWAYS give you more power. nitrous only gives you power for a few seconds.
and of course nos isnt better for your engine, ESP without modification of gaskets and whatnot. all a turbo will do, is put more air into the cylinders at a certain point. nitrous makes your engine burn A WHOLE LOT HOTTER than it is supposed to. thats what it does. a turbo will not make the engine run much hotter, because if you are halfway intelligent, you will have an intercooler.
also, if you are halfway intelligent, you wont put the boost at 20 or something.
the only thing the turbo will change, is the compression. it puts more air into the cylinder. the only way the tranny is effected, is if you try and add 150hp, in that case, you might have to get a better clutch. but you will have problems with gaskets before you have tranny problems. any good turbo kit will come with those upgraded parts though.