It is basically irrelevant at cap. I did testing with light/heavy xbows and repeaters a year or so ago. At level 1 with zero ranged feats, the difference in how many bolts were fired per minute was much better with light xbows/repeaters. At cap, there was no (noticeable) difference in rate of fire between light vs heavy; animation caps or other stuff had made the reload advantages of light vanish.
For leveling, l would use light at lower levels and then stop worrying as one approaches cap. Others might try testing rate of fire as they pick up ranged feats while leveling to see when the exact point occurs where the difference vanishes; I just couldn't be bothered to test to that extreme.
If one is running swash or gnome (or just has a really nice light xbow) then go light otherwise heavy at cap should be a bit more damage.
As for the difference between light/heavy xbow/repeater (but like Steel mentions here and like I said above, once feats are involved things get murky):