In terms of plot, xp and loot it really doesn't matter.
The loot isn't anything you can't get fairly quickly in Chapter 2 - especially once you get the first battlefield key. Ditto on the experience. As you've probably noticed, the lower your level the more experience you get for killing the same enemy. So experience tends to even out. Sometimes being a higher level will cause higher level enemies to spawn but even so the difference between starting Chapter 2 at level 10 or 11 is quite trivial.
Go back to Chapter one if you enjoy wrapping up all the loose ends and aren't planning on replaying the game. Stay in Chapter two if you place a higher priority on progressing through the plot and eventually (this is NOT a small game) getting to the end.
