If Sneak experience earned by sneak attacks is based on number of hits, then it may be more efficient to use a weaker weapon.
Is experience gained based on the damage dealt, or number of hits?
Successful sneak attacks give the biggest skill increases. Melee sneak attacks give 12x as much as ranged sneak attacks. The amount is flat per sneak attack and does not depend on the damage done.
Source: UESP.