I have seen both play too, and I think Tavares looks better at the moment.. Stamkos is more complete though, but Tavares is better offensively..
Better career doesn't mean better offensively. Could mean alot of things. That's why this is kind of a poorly worded question. Could mean conn smythe/cup winner, best leader, best all-round player... etc.
Anyway, I really have no idea as I've never seen any of them play. I will pick at random.
I felt like being a little different, so I went with a kid who's a star in the making and has been a star at every level, Paajarvi. The kid will be a very good player. He's truly a fun one to watch. This is a tough list to pick from though. It's just too easy for me to take Tavares, so I thought about things a lot more.