I am going out on a limb here, but, its been my experience that the longer lens gains you less, than cropping will, sooo, more mp would be better, big lens's are big money. Lot of money going from a 200mm to a 300mm to a 400mm.
And you are right about the lcd not telling you much, but in a dark room, glasses and zooming in on the pic, the lcd will tell you a lot, I can see the difference between pics without going to the puter.
Heres a pic 200 mm from my d40 6 mp cropped.