Basically, it comes down to how much you want to spend.
ABS is cheaper from the start and can be painted, but it looks fine, generally, just by itself - black plastic.
Urethane is more expensive up front and you almost always have to have it painted.
If the style of the lip is say, like mine:

Its probably safe to leave it unpainted.
If the lip is, however, like the ctr lip (98 spec is SO much better IMO):

It really should be painted...because otherwise, it'd look pretty shitty.
$100 now for abs and be done
$100 now for urethane, plus another $100 for paint = $200 (give or take a few $10s)