Originally Posted by 14R
I'm a little biased since I became a big fan of compact folds. if you were talking about 20+ miles a day I would suggest a 20" wheeled bike, probably the Mu or the Speed Pro. Since we are taling about 12-14 miles + a train, I would go with a smaller wheel, probably a Birdy or even a Brompton. You may arrive 3-5 minutes later but these bikes are 1) more convenient to carry and 2) will expand your horizons to take the bike with you on travels since they pack faster/easier than those with bigger wheels. If really compact folds is not a deciding factor for you, I still suggest the Birdy.
Just like other contributors, you don't have to agree with me.
14R, Have you ever been on a Birdy? My average speed is no different on my 105 equipped Birdy than on my friend's 105 equipped road bike. Some on the Birdy forum prefer their folders b/c they feel that they are faster. (Could be mass-speedometer mis-calibration with the little wheels!) Today, there was a touch of a tail wind on the commute to work, and I hit 29.5 MPH on a flat at one point. I've had the Kool Aid, and it's tasty.