Sambo has better takedowns and leg locks - BJJ has a better ground game. You also need to concider which Sambo school you go to as traditional Sambo doesn't allow chokes so some schools don't teach them, which will sometimes lead the sambo guy open to chokes from a BJJer, look up Roy Harris's story about a Sombo freind of his that sparred with Royce once, they were dead even untill Royce figured out the guy wasn't up on his chokes at which point Royce started tapping him out almost at will.
And anyone that uses Fedor as an example of why Sombo is better isn't being fair - Fedor is a freak of nature and it is the fighter (not the system) that makes him so good. by that logic I'd put big Nog up with every other Sambo guy in MMA or talk about how Rickson won loads of Sombo matches and comps to show BJJ is better.
So in closing - both are great, if you have the option you should try both and see which you prefer