My answer = EC135 is best for the IR without a doubt. I have flown both types but have way more time on the AS355, and I eventually completed my IR on it too, (after originally being booked on a 135 IR course) and boy did I make hard work for myself... My only major regret in my helicopter flying career so far is NOT doing my IR in the 135. If you want to pass an IR I wouldn't focus on what you think you might fly in future (chances of co-pilot work on either type is slim as has been said) but which course is best all round, and in my opinion it's the 135, even if it does cost more £££.
Good luck,
BC