40D 0r 600D ? strictly for MP-E 65

Hello folks

First i want to say that i am new on this camp as i am shooting nikon ...dont bash me on that :)... ( not 100 % new tho, i use also the S90 with adapter and raynox 250 & 502... ) ... which brings me to the point:

i decided to build up a macro system using the MP-E which i bought and i am breaking my head now about the camera to use with as i can not afford a 60D, 7D or FF right now....and i have 2 offers..the first would be a used 40D and the second either a 550D or 600D and i would be very grateful about any advice you could give me what would serve me better...

Its not about which camera is more professional in handling, i have the D7000 and know that the 600D is lighter and supposed entry level and the 40D is more sturdy, better viewfinder etc....

i find myself in between this 2 cameras because on 1 side i know that with a resolution of 18mp vs 10mp i have more room to crop which i find very handy, at least a bit cropping can not harm..i could print bigger too, am i right? But i d'ont know about the picture quality between this 2 cameras...resolution aside...
would appreciate very much your take on this!
What would you go for, the 40D or 600D?

thank you!

--
patrick