TT bike, or new commuter?

Due to a bit of a windfall (I applied for PPI on the very last day, thoroughly expecting a polite 'sod off', but it turned out very worthwhile!), I have decided to blow most of it on a new bike.

The first thing I thought of was a specific TT bike as I do a lot of triathlon training and around 6-8 races of varying distances each year (well, most years obviously). At the moment I have a fairly good set-up of tri-bars on a aero frame (Planet X E130) with 60mm carbon wheels and an ISM saddle. I was reading in certain places online that the position a dedicated tri specific frame puts the rider in means there is less fatigue felt when going into the run section of the race (and as a cyclist first, triathlete second, I could really do with this), but then I have also read it makes flip-all difference. For me buying a TT bike should mean I would improve the bike-to-run efficiency and hopefully the comfort whilst in the 'tuck' position - I have an Iron distance event next summer (was supposed to be this summer...), so comfort is another important gain.

I then wondered if the gains of a specific TT bike were so marginal, then perhaps the money would be better spent on a new, lighter, better commuting bike. After all it will be used far more often than a TT bike, so £ per mile far more cost effective. I was looking at, again, Planet X and their ranges of disc brake bikes, and they have both carbon and Ti full bikes in my price range. At present I have a second hand Cube Cross bike (Alu), which is perfectly adequate but a bit of a lump, and I commute in the Lakes so not flat by any stretch.

After writing all that I'm really not sure what my question is, but I suppose I'm trying to get opinions on whether a TT bike would be a good investment for triathlon, especially as I will probably be doing more medium and long distances in future (I could even try some local club TT's as the local club open them up to non-members). Or, as I have been thinking, a new, lighter (and lets face it more bling) everyday bike for commuting etc. And then of course recommendations for whichever you think would be best - budget around £2k

A TT bike would need to ideally not have the brake calipers tucked under the BB, as I really don't think the aero gains outweigh the awkwardness of maintenance and the extra road crud they would pick up - although I would consider them if the bike were perfect in other respects. Frameset only would also be an option, as I can bastardise the EC130 to a certain extent, especially the wheels and groupset.
A commuter would need to take mudguards, have hydro discs and be fairly light.

Thanks.