While you are correct that the value will be lower further down the road, the car won't lose value as quickly as the Merc. Depreciation hits hardest in the first few years of ownership, and tapers off as time goes by. In addition, luxury cars lose their value much faster than other cars. Look at an S600 if you want a steep depreciation curve. The AMG badge will help a bit on depreciation, but less than you'd hope.

What that means is that financially, as you've said, the best move to make is to keep your crossfire. Going with an SRT-6 would also be a better option than getting a C55 (the depreciation has already slammed the SRT-6, while the C55 has a bit more to go IMO).

That said, this is assuming all things are equal. They are not. As you mentioned, fit and finish, interior space, perhaps the V8 engine note. In the end, it's all about your priorities, and if you feel that the C55 is worth it, then go for it. You might as well enjoy your youth while you still can. And house prices aren't going to go up like crazy for a while (you should buy a house because you want to live in it, not because it'll go up in value IMO... just like cars).

I personally only buy used cars, and I try to make them last as long as possible before getting another one. It's a lot easier to do that if you buy a car you love.

If you're gonna go for the C55, I'd haggle the price down a bit more. I fear the salesperson probably didn't drop the price much perhaps because he felt you're in love with the car.