If you have small hands or your carry space for a handgun is on the small size, the 34 has things going for it there. Otherwise, the 18 is the winner hands down for 1st to acquire. The 34 will need bigger grips, such as the S&W target wood grips or the S&W banana grips, for someone with any hand size other than petite. I don't like the banana grips for centerfire J's but find them a winner on the 34.
Regardless, whichever of the two you find first, in the condition that you desire is what you should lay out bucks. Of the 2, I think you will shoot the 18 more, by a big majority. The 34 is more akin to a carry much, shoot seldom.