Colin,
Welcome along to the light side
Technically speaking, I can only speculate by extrapolating my S1 experiences, but here's my 2p...
Robin rates the S2 111S package pretty highly, IIRC...and Ali Crozier really enjoyed wringing the last few tenths out of his

If I were in the market right now, that is almost certainly what I'd be looking at. It offers a great blend of performance for trackdays should you wish, with a bit of "luxury" (!) with a taller 5th gear for commutes and longer tours. And with 156 relatively reliable bhp on tap, in an 800kg car (or thereabouts), it's none too shabby.
135...well...if it's a "135 Sport" in S2 guise, then it's no more than a simple head conversion on an otherwise std 120bhp S2. Which is fine as far as it goes, but it's a simplistic head job, and a Kiwirog Sabre head (specced much like mine...details on request) is a better result IMHO.
However...if you are looking at the pukka 135R, this is an entirely different beast. Still the same CNC machined 135 head as from the 135 Sport, but with Lotus motorsport suspension (=HARD compared to the already fairly firmly sprung S2), OZ rims and semi-sticky tyres. Plus pimp-spec interior, and some serious limited edition kudos. I have driven Robin's 135R a few times, albeit not on track, and as a road car it's great but VERY firmly damped. Well damped, but pretty hard all the same. Might wind you up across rough roads, but if your beaten track is reasonable tarmac, then it's a beaut. And if you want to drive it to a circuit, abuse it for a day, then drive home with barely a whimper from it, then this is a strong option.
You will have fun choosing and as you can see, no end of opinion available on here!
If you tell us a bit more about how you expect to use it, we can maybe help a bit more! Note, you may find yourself using it in different ways to those you expected once you get it. These little plastic cars are addictive.. surprisingly light on fuel (even on track), relatively self-maintainable (with Jedi Master Iddon on tap to keep us right) and relatively depreciation proof if you buy into the right spec.
Enjoy the choosing
Campbell
(S1 11 years veteran and counting...can you believe it!...)