This sounds like a homework question, but I will give you a hint. Caffeine is an adenosine competitive inhibitor in the CNS while nicotine is a cholinergic at nicotinic receptors in the CNS and SNS. Look at what those mean for differences. For similarities, I would definitely look at their metabolisms and possibly similarities in their effects (although this is a stretch, nicotine effects depend on dose more so than caffeine).