FlyerTalk Evangelist
Join Date: Dec 2003
Location: Benicia, California, USA
Programs: AA PLT,AS,UA PP,J6,FB,EY,LH,SQ,HH Dmd,Hyatt Glbl,Marriott Plat,IHG Plat,Accor Gold
Posts: 10,820
The consensus is that Delta has the worst FF program in terms of difficulty actually using miles you accrue for free flights, so depending on which airline best suits your travel pattenrs you might concentrate your flights on AA or UA, whose programs are much better. (Also note that UA and CO are merging and that US, like those other airlines, is a member of the Star Alliance, so if you are a member of one of those airlines' programs you accrue miles if you fly the other ones. But stay away from US, which is the pits.)
I agree about the spg Amex card being the best, all things being equal. The only caveat is that you might want to concentrate your hotel stays in one chain. If it's spg, then of course the spg Amex card is a natural for you. But if it's Hilton, then you should get a Hilton Amex Surpass card. There's also a Marriott Visa card if you go that route.
The point of concentrating on one airline and hotel chain if possible is that it gets you status that translates into extra miles or points, as well as upgrades.
Don't know about the giftcertificate.com thing.
Hope this helps!