Reviews.

But I am aware of the bias in the industry (OMFG It's GTA! Let's give it another 10!)

I don't trust either but if i had to pick one i'd go with reviews.

Sales. Absolutely sales. Not even a second thought about that. All the legendary games became legendary not through amazing reviews but through sales. If a game you don't like sells amazingly well, it's time to reassess why you don't like it. If a game you like flops, it's time to reassess why you like it. Sales are facts. Reviews are opinions from so-called "experts". Unless by reviews you also mean customer reviews which I tend to trust them more than publications' reviews.

OMFG CALL OF DUUUTTTYYY!!!!!!! 4 HOUR CAMPAIGN!!

reviews

i trust reviews a bit more than sales. Though i disect reviews to see from their experiences if ill like it.

Reviews obviously. Lots of reviews from not only websites, but from friends who have played the games as well. Some perfect examples are the horrible Iron Man and Harry Potter games which sold over 500K copies. Whereas games like Beyond Good & Evil and Pyschonauts. sold worse, but were much better games. People who buy games because of sales along will miss out on a lot of great games and could end up with a bunch of stinkers.

Reviews definitely. These are in depth analyses from people who've actually played the game. Remember how we always say sales =/= quality. Well, reviews try and measure the quality of a game.

If anyone says the trust sales more, I retort with Wii Fit

Wii Sports: greatest game of all time

Do you doubt it? Perhaps. Perhaps I doubt it too. Perhaps I also doubt Super Mario Bros, Tetris, Duck Hunt etc. Did everyone think of them highly when they were first released? I doubt it. We won't know what way will Wii Sports go. Not yet, at least. Perhaps in 10-15 years, we might have a better idea of Wii Sports.

And here is some extra food for thought. Highly regarded games get a LOT of clones. Super Mario Bros, Street Fighter II, Tetris, Grand Theft Auto and yes... Wii Sports.