it depends on what your games are. You need to check the base trade value of the games at both places and then compare how much you would get with the bonuses. The 100% at Best Buy sounds good, but they drop base values during the 100% promotion and it's often less than the value with the 'normal' 50% bonus.
The Gamestop deal requires that each game trade in for at least $3, which a lot of older games (especially sports games) do not. If all of your games do qualify, you might need to separate them into two trades, one with 7 to get the extra $35 and one with 4 to get an extra $10. The register might do it in one transaction, but it might not. Just check the display or ask the sales person to make sure what bonuses you are getting.
A DSi trades in for very little money right now, but compare that too.
Just do a little research and keep your options open. You can trade in at Amazon too, and if you can get more for some games at Best Buy than Amazon, keep in mind that Best Buy sells Kindle cards which is the same as Amazon credit. I think Gamestop sells them too, but haven't actually seen a Kindle card in a Gamestop yet.
Bottom line answer, there is no best place.
The Register will automatically do the best promotion for a customer at GameStop one of the coolest parts about the systems.
Source: I do these transactions for customers just about every day and it hasn't had a hiccup in a while (for me at least)
Also most GameStop's should have $25 and $50 Amazon Kindle cards now as well as $25 and $50 Shell Gas cards.
Thanks. My only concern was that the best promotion is just one promotion. If the guy trades in all 11 games, I'm guessing he'll probably just get the extra $35 (which is the best promotion), but if he separates into two transactions, he should get $35 for trading in 7 and then another extra $10 for trading in at least 3 more. That's assuming they all meet the minimum $3 trade-requirement. I don't think the system is smart enough to do that, but I could be wrong.
I agree, the register system at Gamestop is generally pretty smart about giving the best promotion (but it's pretty bad at other things, like tax, refunds, mixed payment methods, trade cards vs. gift cards, etc.), but don't rely on it to do the best for you if you happen to qualify for more than one promotion.