Welcome to FT, unodoshol!

You've learn much by reading, as bkramt1 suggested. It also depends on where you'll be doing your traveling. If you travel primarily from A to B, program #1 may be better, but if you travel primarily from A to C, program #2 may be better for you.

If you're looking for elite benefits, it is best to stick with one program, but if miles and/or price are a main objective, you can belong to many. (I belong to 5!) Just sign up for each before your trip.

Also remember, many airlines belong to alliances. So even if you are flying US, you can earn miles on UA or (IIRC) LH. Likewise, if you are flying on CO, you could earn mileage on DL, NW or AF!