I am only an expert on Delta, being a platinum million miler (working through my second million). Frankly I wouldn't recommend Delta, because they make it to hard to use the miles when and where you want. Some airlines might make it easier.
I can advice two things however: 1.) Stick to one airline frequent flyer program, as it seems like you are doing, but it should be also dependent on where you live - because it makes no sense to go with Delta for instance if the city you live in is not a Delta hub. If you live in Atlanta, Delta is a no-brainer.
2.) Use this site as a reference, they forums for each frequent flyer program. Most of the registered users are heavy duty road warriors and they know the programs. Read some threads, look at the complaints and the good features. The flyer talk forum:
Airline Programs - FlyerTalk Forums
In summary however it appears a few trends are occuring for frequent flyer programs:
1.) As profit margins with airlines slim down, the frequent flyer programs are deminishing in it's usefullness for the consumer. Expect the trend of requiring more points to get free flights.
2.) Airlines are continuing the trend of monetorizing it's frequent flyer programs. That means it will be tied into profit generating efforts more and more - you get ff miles from credit cards and other purchases (essentially the airlines sell their FF miles to retailers). This is good and bad, you don't even need to travel anymore to get FF miles, but this will have the inflationary impact of again, deminishing the value for frequent flyer miles. For flying, most airlines are now tying in the amount you pay for a ticket to how much points you are awarded. So if you are just a casual flyer looking for bargains - less miles. If you are a business traveler that frequently gets last minute tickets at high prices (that would be me) - more miles.