well i pre ordered through amazon for $48 but $30 is better so will cancel my pre order through them. will
this work trading in madden 15 ultimate edition for the x1?
As has been mentioned, for most situations, GCU is a great deal, and you should consider getting it. If you do not, however, then whether you buy from Amazon or BB is pretty much a wash. BB costs less out of pocket, but Amazon is technically $2 cheaper, unless you can't find sonething else at BB to spend the $20 GC on.
Either way, you are spending money + your copy of Madden 15, but if you keep your Amazon preorder, your cost is $48+tax, and you end up with the game and a $20 BB gift card. If you cancel your Amazon preorder to get it from BB, your cost is $50+tax (you can use the $20 gift card as part of your payment, but you still pay tax on $50). So it really depends on your situation and if you'd find another use for that $20 BB gift card.
If you can afford the $30 GCU membership, though, get it before you trade (I think you may be able to do it in the same transaction). It'll get you $2 more on the trade, and save you $10 more on the game, so you'd only be out $18 extra for now, and you'd have 2 years to recoup that and save much more.