I have both a steel and aluminum Swift. I originaly set the aluminum frame up as a geared bike but now run it as a fixed gear. When I bought the steel framed bike (from HRM) it was my intention to convert it to a fixed gear. The only reason I bought a complete steel bike is because the cost of a steel frameset was only about $100 less than a complete bike. I posted some weights on the "Making a Swift lighter" thread so, if you're interested you can look there for the weight differences.

I can't feel any difference in the way the bikes ride and handle. I have different tires and saddles on them so, no comparison of ride quality would be fair anyway. One thing that is different is the steerer tube set up.

The aluminum framed one is 1 1/8" OD clamped on to a welded adapter in the fork. It also has a roll pin that will catch a notch in the fork so that if the quick release failed you could still steer the bike as long as you didn't pull straight up on the bars.

The steel framed steerer tube is two piece with a 1 1/8" OD tube telescoped on a 1" OD tube clamped to an adapter in the fork. You can adjust the handlebar height by sliding the outer tube up or down. The adapter in the fork is aluminum and while I have not had it apart to confirm this, it appears to be a quill stem that's been modified to accept the 1" steerer tube. There is no safety device like the roll pin on the aluminum framed bike. If the pinch bolt fails (no quick release) you won't be able to steer.

The steerer tube on the aluminum framed bike is slightly stiffer. The aluminum frameset is available for a reasonable price. Other than this slight difference in steerer tube stiffness they feel the same to me.