I would recommend you rent a 357 magnum at a range and shoot it with 158 grain magnum rounds before you think of buying one.
It just has a ear crunching sound that gets really tiring to shoot even with hearing protection on.
Of the two,I'd actually recommend the 45 acp in a revolver as it is a much cheaper and readily availible round and there are way more options for shooting the 45 acp then the 45 Long Colt round.
Plus Smith makes a way cool Performance Center Model that is like buying a Grand Sport Corvette to shoot with.
Plus the 45acp revovlers use moonclips that act like the ultimate speedloader setups for revolvers.
And there is this guy that shoots one pretty fast.....hmmm what is his name......
However,the 45 LC is a legendary round and if you simply must,you simply must.
I'm not a fan of the 357 round even though I owned a great Dan Wesson revolver for several years that shot it.