what a great dilema
I think the first thing you need to work out is - will it be used for racing, or is it just for trackdays and fun? This would have a huge bearing on my choice.
Atom - awesome car - great fun at track days primarily as it doesnt like going in a vaguely straight line. You dont need the 300 either, as it cant cope with 245 let alone 300. Part of the reason most youll see on a track doddle through the bends and explode down the straight. Loved mine, but you are almost wearing full biker gear every time you use it, and its rarely if ever warm! It uses the uprights, and bearings, from an S1, which are gash, cant corner at the same speed as the 2-11, unless you take very creative lines, but if you like sideways, its so much fun, and with practise very controllable. As Walshy put it "this is your ultimate car!", due to its sideways from start to finish tendancies. Almost too fast for KH trackdays though!
2-11 - didnt like it much the first time i drove it (hungover), but the 30 mins i had on track with Scottys at Spa had me totally sold. A tremendous track car, but in reality its a race car. Not as adjustable as an Atom or Caterham (input wise mid corner) and doesnt deliver to my mind the same trackday thrills, but its easier to pedal and fantastically fast. It really does show just what can be done with that chassis. Taking some of the more challenging corners at Spa flat was just epic, as was Eau Rouge (it was too fast to go flat)

You will be destroying everything on track, and constantly waiting for others and being quickest can get a bit dull.
R400/500 - might even be a chance that my mate might sell his R400 (Duratec), which is only 18months old and get an R500 for the sequential shift. Awesome car, and hugely adjustable through corners, limpet like. If you havent tried one you should. I really enjoyed bringing this one up from London, the A68 was a riot, and it is even faster now accoridng to its slightly mental owner

Cant wait to try it on track. Have driven a few on track and it was from that i decided to buy an Atom.
Exige - just doesnt give you the raw thrills of the above (as you fell further from the action, plus point is refinement and long distance cruising ability though!), but a fantastically accomplished it of kit. The best one I've driven to date is Ali C's which was hilarious fun around Le Mans. Huge brakes on Alis, Supercharger makes for effortless on road pace, if not hugely brilliant on track once the heat soak gets in to it.
340R - just bought one, and love it so far. Seems to be a bit of a compromise between the Atom and Elise. Much lighter, feels sharper, and looking forward to getting it on track asap! Loved the others i have driven though, and unlike most of the other cars on here, i suspect it will actually appreciate!
Wouldnt bother with a Prosche if its not your main car.