10mm if you like semi-autos, .357 if you prefer revolvers. Or something like that. With a glock 20 or 29, you still get more rounds before reloading. With the .357, you get a platform with more versatile barrel length. Love both, but haven't seen the .357 do anything that the 10mm couldn't.
However, here's the kicker: .357 is much more popular and accessible...and you can shoot .38 in the same weapon for less money. With 10mm, you are stuck with the price, period. Your only way out is to have a glock or Witness and get yourself a .40 barrel so you can shoot the cheaper ammo. Then again, that takes away from the purpose of having the 10 mm in the first place. For longevity, go with the .357, it will always be around. The 10 should be around too, but weapon platforms are fewer and it may actually fade more into the background.
Of course, you could always get both. However, if I had to pick only one or the other: .357, for all the reasons stated above. You can always hand down a good revolver from generation to generation, too.