e-heath, I just got the Capital One Venture Miles card and I love it... allows you to use your miles with your airline (i Fly United out of Denver / for work a lot), and anything over and above what I can't use my United miles for I use my venture miles. Allows for some really good flexibility with flights, which is nice.
AMEX would probably get you more points / miles, but the lack of flexibility can be frustrating, and with AMEX, you have to pay off the entire balance every month. This isn't an issue for me, per say, but it can be annoying in the event you have, say, an unexpected emergency expense and you have to still pay off your credit card.
My 2 cents. I'd go with the venture miles card, Fly Delta all the time, and rack up a preferred seat with them simply by using them.