Check your existing motor name plate. Totally depends on what you are going to use the motor for.
I find those 2 pole (3450 rpm) motors sort of a pain. They seem to be too fast for most of my applications. Of course, you can belt it down, add an intermediate shaft etc. - but when you can easily cut the speed in half by using a 4 pole motor (1750 rpm), then you don't have to step the speed down as much. When I can find a 6 pole motor (1150 rpm), I grab it.
For example, on a conventional post style drill press, I always find even the lowest speed to too fast (a common thread topic here). One solution people use is to set up and intermediate pulley - which works great and can get you a very low speed, though it is a bit of work. Another very simple solution is to mount up a lower speed motor - the speeds won't be as low as having an intermediate pulley - but very easy to impliment. Of course, for the same horse power rating, the lower speed motor is going to be larger in diameter (has to make up the horse power via more torque instead of speed)