I have see that on Steam there are a lot of games for Mac (multi-platform), do you think that is better buy on Apple App Store or on Steam? I never buy on Steam but I had see that the price are a lower respect to App Store.
I boy from both. I thinks Steam is more portable than the App store as you can also get games to play in any other computer OS. Steam provides great discounts almost every week, and many free weekends to play and try games. I like steam.
Steam!!!! Although I am a big fanboy of steam.
It's cross platform, and many steam sales. Many games I've bought on steam have been because of very low prices. I got NFSHP normally $70 for $5 during a steam sale.
Steam is often cheaper, and as noted the games are cross-platform. On the other hand, it's often buggy, and you need to launch the Steam client to play your games (they don't go into the Applications folder like software from the App Store).
Play with others ( App Store only allows you to use the Game Center for multiplay AFAIK, which renders games like Civ5 for example virtually useless )
Mod community with installation
Faster updates AFAIK
Alpha programs, beta suubscriptions etc.
Steam sales, third party website who sell cheap Steam games, gifting ( to and from Windows/Linux users )
The disadvantages are:
Buggy and sometimes slow client
Connection issues ( rarely )
As you can see above, the advantages certainly outweigh the negative aspects of Steam. The mac app store is - imho - really terrible when it comes to games ( and certain kinds of software ).
The best thing is that you can play with others and switch platforms almost seamlessly. Just log in to your account on another pc and you're good to go. The MAS doesn't provide anything like that.