Generally speaking an Evo WILL cost you more money in the long run than a cossie. How much, depends on how much your going to spend on an evo. The newer the evo, the more you'll lose in a short period of time.
Ok if your comparing a £5k cossie to a £5k evo then they'll probably be similar.
As an example i was looking at evo 6's 18 months ago at £13k. The same cars are now going for around £8k!!! Thats around a £5000 loss in less than 2 years.
If your looking at evo 8's and 9's, especially the top models, then your going to lose thousands of pounds.