After all those high-powered devils, it's nice to review a weaker kind for a change, don't you think?
As most people here know, imps are so overwhelmingly similar to quasits that I'm already having trouble justifying a different LA. Then again, I may just get surprised by an overseen ability.
The imp has slightly better strength and considerably better charisma than its demonic equivalent, as well as superior NA. The quasit has better natural attacks, but that advantage disappears as soon as both of its claws are occupied. The slightly higher fire resistance is notable, but not enough to make up for the difference.
SLA's are almost equal, with the imp getting Suggestion instead of Cause Fear. It should be obvious this ability is significantly more useful, especially as foes start getting more HD. Alternate forms seem about equal.
To summarize: the imp is definitely stronger than the quasit. However, this power difference is not large enough to warrant another point of LA. While this may make imp PC's more attractive than quasit ones, there's still reasons to play the latter.