I'm surprised to be disagreeing with Cosmoline. Although light loads for .44mag don't stretch cases, the round inherantly pushes most that reload for it to push the limits. Sometimes before they're ready. Is it easy to reload for? Yeah. It's big enough to handle and when you look at a batch of loaded rounds you know you've done something. But just as I've started my boys out with rimfire guns, I surely didn't hand a big magnum to my wife when it was time to teach her to shoot.
Until I saw that you're not a wheel gun fanciear, I was going to suggest a .357mag, but that you start by loading .38spl for it. Even warm loads, or loads with a smart powder, (one that a double charge won't even fit in the case), it does lend a lot of room for 'inaccuracies'.
I don't think I've ever had to trim a .38spl case. I know I've never trimmed a 9mm or .45acp case. Back to low pressures.. The .45acp is probably the safest. Yes, it is big enough to grab ahold of, where-as 9mm and smaller can be a pain to manipulate. As with .44mag, the .45acp uses a large pistol primer. Not that you handle them with your hands often, they are easier to see when you spill them. Sorry, I'm just not a .40 guy so I won't comment about them. 9mm brass grows on trees. .45acp brass is best scrounged by digging to China.
If I was the firt post, not following the advice of our peers, I would have and still suggest the .45acp, and especially a Colt 1911. Which one? I suppose that's for another thread.
-Steve