Cyclocross - Which frame material?

I would like to build a cross bike/commuter bike, used mainly for cruising around, no real cross racing, just fun. I'm unsure about which frame material would be best. I could buy something like a Redline Conquest Pro alum frame, or buy something like a Felt F1X complete for about the price I could build one (with Campy Centaur or my current Chorus stuff as I replace it). Would an aluminum frame be too rigid when riding broken sidewalks and parking lot potholes? I was also considering something like a Soma Double Cross frame, with either the steel crown fork or a CF fork. I would also plan on putting a rear rack on it as well. Any input would be helpful. I'm in South Carolina which is pretty far removed from the cross world.

Aluminum is perfectly acceptable. Wider tires will decrease the harshness of the ride more than a different frame material. By using a CX/touring style frame, you'll easily be able to use 30mm wide tires (or more). :)

Whether a frame is harsh has more to do with who designed it, how and for what than the material. For what you're talking about the only thing you don't want would be an expensive race-oriented frame of any material. Doesn't matter if it's all CF or steel or what, a race bike (even a cross race bike) is rather unpleasant on bad road. My very old aluminum Redline is no more harsh than my steel road bike. Cross race bikes ride nicer than road race bikes, but neither is really suited for the riding you describe. Unless coffee shop cred is at stake, in which case I'd say change the game.

In the last 20 years or so road bikes have adopted imitation crit racer geometry and as a result a lot of cross bikes have been designed and sold for people who just want a classic road and all around bike instead of some buzzing lightweight that's fast as hell, punishes you on every RR crossing and won't take tires over 25mm wide. That is probably the sort of bike you want one of the cross bikes with fender and rack bungs and a slower geometry. You know, like there's room between the seat tube and the back tire.

Soma is said to be a nice one. There are a lot of good choices out there. Don't get locked into what you see online.

Not so much for the comfort factor (you can have a comfortable frame made of any material) but I would choose steel for any non-racing purpose. I also like barcon shifters fr non-racing applications. Durable, light, and smooth. That's just me, maybe.

The surly crosscheck complete meets both of those criteria, and will take a rack. I would avoid the Felt and Redlines, from what I understand they are primarily designed as race bikes.

Realyl though if you're just on-road you don't need a cross bike at all. If you want to build frotm he frame up a surly pacer, soma smoothie or smoothie ES or Salsa Casseroll would all work great.

Tires size and fork/frame geometry affect ride quality ENORMOUSLY more than the frame material.

I have a Jamis Ventura Elite - an alum bike with carbon seat stays and carbon fork. I used to ride 23mm tires and switched to 28mm. HUGE difference. It seriously felt as if I was riding a different bike. Combined with a slightly lower seat position it is now very comfortable!

I say remove all doubt and drawbacks and just buy ti. Oh wait, the drawback to ti is that the decals don't stay on well. Oh yeah, and they're kinda pricey. See Habanero.

Bamboo.