Aluminum vs steel vs carbon

I have aluminum MTB and tri bikes, so just my $0.02 from reading a bit:

Steel = heavier, and more comfortable

Aluminum = lighter, and least comfortable

Carbon = lighter, and more comfortable

Of course, this can depend on the bike.  Some high end steel bikes may not be that much heavier, etc. etc.

If the majority of your riding is in a hilly area, the weight can make a difference.  The flatter the courses you ride, the less weight comes into play and the more aero is important.

You won't find many new steel (or CroMo) bikes being sold unless they are 20+ years old or from some custom / specialty manufacturer. Steel rugged but heavy and will rust eventually.

Aluminum is much lighter than steel. It has better impact resistance and its repairable (unlike carbon, which you can't weld).

Carbon does not make you faster. Its a little lighter than aluminum (typically a few hundred grams on the frame and in the wallet areas).  That weight difference won't matter much unless you are riding big hills and are already down to 5% body fat and 2 EPO injections/day .   A lot of people say carbon is more comfortable to ride on for long distances but personally I couldn't tell the difference.

These days most $2000+ frames are made of carbon. Most frames are made in China and with low labour costs it's just about the same as to properly weld a frame. 

If you are a MOP rider on a budget, an aluminum frame will not hold you back any.  Of course it does feel nice to have the latest and greatest piece of equipment, and for emotional satisfaction, carbon is king.

I wonder if some manufacturer will come out with $20k frames made of aramid fibre.

Will somebody find that post, please?  I'm lazy tonight.

Not exactly:

http://velonews.competitor.com/2011/10/bikes-and-tech/carbon-repair-options-for-repairing-carbon-bike-frames_196178

SYJ

Sorry charlie, aluminum isn't really repairable.  If you don't have a replaceable drop out in your AL frame and tweak it, congratulations you just wrecked your entire frame and will need a new one.

unlike steel, aluminum isn't very malleable, i.e., you can't bend it back and forth without major stress damage and fracture.

The biggest differences in the materials are weight and how they can be manipulated and shaped on the bike.  Carbon offers the most flexibility, which is why it is often the material-of-choise on higher end bikes.  But, a good bike can be made with any of the materials (plus titanium, or even bamboo!).  "Comfort" depends much more on how you fit on the bike, the tires you choose (and, importantly, the air pressure), your saddle, and the roads you ride.  Carbon can be designed to be more 'forgiving' (vibration dampening) than aluminum or steel, but it can also be made to be just as 'harsh' (or harsher).  It all depends on what the builder is prioritizing. 

In short, you generally don't want to choose your bike based primarily on its build material.