Better, I'd pick the one I have, the 21sf.
Better as a first gun, the 34, by far.
The 21 takes a LOT of practice to get decent with. 45acp, is a powerful round for a gun as light as the glocks. I'm sure the 34 takes nearly as many rounds down the pipe to get good with, but being as it's heavier and a lighter calibre, which is cheaper to shoot, it'll definitely save you a ton of money on your way to getting good. Also, in theory a longer slide makes aiming easier, ymmv.
Once you master the Glock 21, though, you'll be very happy with it. The only way to go up from there is a Glock 20, or to bump up to 45 Super. Not nearly enough 45 Super talk for my liking, as it stands, you're pretty much on your own with that one.
Unless of course you want to play organized shooting games, for ipsc or idpa, you are way better off with the 34.