Its not all about bhp which is a peak power figure, its about the amount of torque that moves you. 2.0 will always be more and easier to drive becuase it kicks in lower down rev range.
When I bought my valver over 4 years ago I thought that the only difference is 13bhp and several insurance groups.
With a BMC CDA, Magnex inc Decat, BB Chip and Supersprint Manifold it will be pushing approx 150bhp or more (last RR was 148 without manifold). So I may be in the range of a Williams bhp wise but doubt I will be close in torque. If was to race one still think the Williams would have teh edge due to power coming in lower down rev range. I would have to be at approx 4k and above whilst they will get same from approx 2k. So easier to drive a Williams.
Most get Williams engines fitted as straight swap. Due to low number of Williams the engine goes for a premium. It is cheaper to get hold of a Megane 2.0 coupe engine and just use the bottom end matched to your Valver engine, but only cheaper if you were going to be stripping down new engine anyway before fitting. If you wanted to just swap straight over, Williams engine is cheapest of you find the odd one on here or ebay that can go from as low as 350 quid up to anything close to 800.
If you are doing either, you would be best to strip new engine as all the expensive jobs like cambelt, water pump, clutch etc which are a bugger to get to can be done with engine out. So depending on mentality you either just do a straight swap and hope or you take apart new engine and check all bits and repalce where necessary, then you should have peace of mind and many miles of trouble free motoring.
but im new here...anyways
