I don't see any reason why playing in different positions or styles or teams means you can't compare players. For instance, Wayne Rooney's much better than Gareth Barry... that's not controversial. He's just clearly better at football, which is reflected in things like price-tag.
It's possible for two players to be roughly equal, of course, and in your view that's the case with Tevez and Torres, but I don't see why you need recourse to the idea that they're like apples and oranges. They're both footballers.
I've not seen that much of Torres, but from what I have seen he looks better... quicker, more dynamic, more dangerous, he just looks the business, a top-class striker. Tevez is a very good, strong, clever player though.