heya,
mopho and evolver are quite different beasts. while mopho is concentrating only on obtaining classic analog sounds(with the help of the sub osc) - deep basses an stuff, the evolver is much complexer, due to its routing possibilities and hybrid character.
i suggest you take a closer look at the characteristics of these two synths. you'll find everything you need on dave smit's web page:
http://www.davesmithinstruments.com/
and it's equally important you take a look on youtube to see those beasts in action.
anyway, it's just a mater of choice. of what sound you really like. if you're more inclined on mopho-side you might consider a tetra as well. it's just like 4 mophos but without the external imput feature.
good luck mate!
:)