I don't think it makes sense to compare these products directly. Samurai is a gateway only -- you still need a merchant account which is a lengthy process, while Stripe lets you start accepting payments immediately. That distinction implies they are for different use-cases. Samurai is useful if you want a simpler integration than Authorize.net and Strip if you have nothing, and want to get up and running immediately.

The simplest example of the difference is the sign-up process, Samurai requires you to enter a company name before you can start using the service -- Stripe doesn't even need an email address.

Another key difference is that (from what I can tell) Stripe handles all PCI compliance, whereas Samurai still requires you to handle some aspects.