The megane RS looks great from the outside, on the inside its a bit disappointing and the performance doesn't come near the M135i. The chassis in both cars are being praised as phenomenal.
The Astra OPC is great. Looks great from the outside and acceptable from the inside. But don't forget it's FWD (if I'm not mistaken) and the M135i is RWD, and has more power and a better chassis.
There was a recent review by Chris Harris comparing the megane RS, Astra OPC and the focus ST. The focus won, and I'm sure the M135i, as a package, and for the price (only a couple more thousand pounds), is still the better bargain.
That's just my opinion on the matter