Start with background, ending with question.
I'm in a bit of an odd predicament. I completed (woohoo!) my first long-distance commute last thursday, but the trip left me with some concerns. The trip is 35 miles one-way, and about 2/3 the way there, I was up on the flats of the road bike searching for that magic posture that would NOT amplify the pain in my shoulders. I made a point of stretching after I got to school (my last ex-girlfriend is a massage therapist...I promised I'd start stretching), and by the time I was ready to get on the bike again my back had quit complaining. 10 miles into the 35-mile return trip and my back was where it had left off with the pain creeping down my spine into my lower back. Not Fun!
At the beginning of this month, I discovered that my road bike is too small for me by a standard frame size: it's a 58cm and according to (very basic) fit calculators I should be riding at least a 60cm frame. The stem is at-a-glance noticably below my saddle. If I keep biking to school, the gas savings can get me a new frame within 2 months. ($7.50 per trip, allowing for 1 driving day/heavy cargo & supplies)
I was using this bike to commute because of the wheels/tires: 700x28's
I have a 21" mountain bike that I got rather recently, with slicks that I got with it because I couldn't shake the feeling I'd need them. The slicks are 26x1.5"
The Question: Should I keep riding the road bike and just find a way to put up with the back pain for a couple of months or should I take the mountain bike with slicks? How much slower will those 26x1.5's be compared to the 700x28's?