There are lots of variations on the theme. You have choices as to the quality of the bearings which will influence the run-out of the center as it rotates. You have choices of 2, 3, or sometimes even 4 row bearings which will influence the side load capacity. You have choices of extended nose or standard nose which influences the clearance for tool posts as you work near the end of the workpiece. I believe I've seen live centers with a spring-loaded center which would help make sure that the center maintains positive contact with the workpiece.
I'd say that you should buy the best one you can afford. If you're using a Quick Change tool post, you'll want to seriously consider one with the extended nose to get the bearing housing away from the end of the workpiece. A live center with a short nose is guaranteed to bring up a fight over real estate near the tailstock.
