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Both are great for jerky, though the eye of round is perhaps a bit more popular (probably because typically it's a little cheaper). Which ever you choose, make sure you get one with the least amount of fat possible (fat turns rancid after a while). Also, have the butcher slice if for you, will save you a ton of time and they will usually do it for free. Let us know how it turns out.
for the do it yourself person eye round would probably be easier to work with it's firmer then the sirloin tip easier to slice, or freeze them up some. just a thought
I ended up using eye of round, and as suggested by old smokerdude, I had them slice it up. Only regret their was that they were not as careful timming the fat off as I would have been. It looked like he cut into the lean meat 1/4" in some places, but probably wasn't more than 1/4-1/2 pound of lean meat total. I was surprised that the 6 lb roast ended up being about 4.5 after being trimmed.