According to THIS, the sirloin tip comes from the round, whereas the tri tip comes from the sirloin.
I'm no expert, but since they come from totally different places, I'm not sure they'd be very similar.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team