The guy at Rotashop.co.uk said to me they can't do ET23's (the offset on the car pictured in here) then told me the offsets they do do, then sent me the pic of car in here with ET23's. To me that is NOT being helpful but in fact deceptive.
Would be nice if AJD S3 could respond but he's not posted since Sept '12
marlow77, sorry to revive an old thread, and furthermore, apologies for the delayed response.
I actually sold the car around October time last year, and came across this thread whilst having a look around the web to see what had become of it since! Seems I really did start quite a craze with the old Rota setup!
In answer to your question, ET23 is the best, and in my opinion, the only way to go. You gain greater concave, plus the aim with custom fit alloys is always to keep the actual wheels themselves as naturally close to the hubs as physically possible - so in other words, use the smallest spacers possible, or better still, don't use spacers at all.
ET23 offset versions drastically reduce the size of spacer required for a suitable fitment, whereas ET38 offset versions will require larger spacers. I actually bought my wheels from RareRims, who had no trouble whatsoever supplying me with ET23 wheels. I strongly suggest you get in touch with them and see if they can help.
I find it somewhat irritating that Rotashop.co.uk, whoever they are, are passing my work on my old S3 off as their own however! Especially when they don't even know the correct specification to which the car was actually setup!