i use an LX with a sachs PC69 - I used an XT once to get the higher gearing for an alpine ride (I still had to get off and push!) but the only difference is weight - I didn't find it lasted longer and unless your a weightwatcher-to-extreme you'll not notice any difference. I don't know about the deore - i use LX mostly.
so to answer your question - no, £5 better spent in the pub!
LX is constructed a little better, but I doubt there's any difference in performance vs Deore. XT is quite a bit lighter, but that's the only advantage.
Cheap cassette and chain for winter, indeed maybe a multideal, XT and good chain come Spring for a season's worth of racing. If you don't race I don't suppose XT is worth it though, although the aluminium carrier does look nicer :) But the only places you save weight between groupsets is cassettes and chainsets, and perhaps V's if you're still using them, everything else is very close in weight. Sachs are great chains, no difference in weight, just harder wearing and better shifting supposedly as you pay more, doubt there's little real world difference though.
Definatly SRAM cassette. Beats Shimano into a 'cocked hat' on price, wear rates and weight. Plus the ramps are designed to match with a SRAM/SACHS chain.
Oh and @Jay. Sachs beats Shimano by MILES on the chain front. There really isn't any competition. They are stronger and take longer to wear out. Whether they shift better is open to debate but they do not shift any worse.
Nothing wrong with deore at all. I've also found SRAM 7 & 9 excellent. However, the best performance I have had has been from XT cassette, rear mech & chainset with a SRAM/Sachs chain (PC69?)