What
Firstly the merc has no where near as much kudo (wow) factor as the S5.
Only last week I had my car re-mapped by APS. This had dramatically changed the feel and responsiveness of the car. The car now pulls through the whole rev range, and the engine most definately feels open. Stock it feels as if the engine was being forced to be tame....all the remap did was unleash the engine. It is something that cannot be described well - apart from having a dyno done to show facts and figures, but can only be experienced. Here in the UK there is a fairly large price difference between the merc and S5, but removing the cost factor I would go with the S5 ;-)
Hope this is of some help. If you do a search you will find a couple of reviews on the remaps done here in the UK, all of which I believe are very positive.