Welcome to FT, kicking_cans!

To assist us in giving you a recommendation, where may you fly? Not that I was going to recommend it, but if you would be flying to London or Rio, I would not recommend Southwest! (I know they don't fly to BOS.)

Since there are 3 major alrline alliances (Star Alliance, One World and Sky Team), I would choose the one (or ones) that have flights to where you may be going to. And deposit all of your miles into one account.

The major domestic carriers in each alliance are:

Star Alliance (or *A) = United and USAir
One World = American
Sky Team = Delta, Continental and Northwest

Each alliance has many international airline partners also.

By what I said about crediting your miles is that even though you fly from (say) BOS-TPA on US, you can request to credit those miles to UA (also in *A). Likewise, your flight from BOS-DTW on NW could be credited to your DL account. As long as the airlines are in the same alliance, you can do this. (You could not credit your UA flight to AA, since they are not in the same alliance.)