If this a bike2work scheme bike?
If not, then I would recommend Titanium. You might have to go second hand though, or source a bike from XCAD.
Something like a Van Nicholas Yukon which will take mudguards with 25c tyres and a rack for lightweight luggage is good for commuting - it won't rust due to salt-encrusted roads, it is relatively light and if you want to take it on a club run, it should still hold up pretty well.
For me, this type of thing is the ultimate bike. I have an Airborne Valkyrie frameset in the loft - too much scope for it getting stolen when left locked up anywhere in Manchester or even in the shed - and for all around usability, comfort and pace I would thoroughly recommend it.
I've ridden steel bikes too... they are just a bit less comfy than Titanium and maybe a wee bit heavier unless the tubes are ultra-thin. A good set of tyres can make up the difference between the comfort of Ti and steel.
Carbon bikes are probably ace to ride but I wouldn't trust the material well enough to buy a second-hand one. Unless you are the type of person who can crash or drop a frame and think "oh that's bad luck" before going out and buying another one, I wouldn't bother with it for commuting.
I would think, in the long run, next to running a car or buying season tickets, then it is feasible to run a carbon commuter thinking of the money saved and odds are that over your lifetime you will probably end up buying one, two or maybe three frames having to replace a carbon frame that's been trashed. Unless of course you are racing, clumsy or just very unlucky.
That's my thoughts anyway.