Hey all, after several months in school (with a Lenovo X60s) I can say that the most important factors are:
1) Weight.
2) Battery Life.
3) Reliability / warranty / service, etc.
4) Performance (RAM, HD speed, etc.)
5) That it runs Windows XP (not Vista yet).
Screen and other issues are really not that important, unless you have any vision problems and need to see really big characters. If you can't live without a big screen for your simulations (ehem, 3D games) you can always buy a bigger monitor or a TV to double as monitor and TV.
My screen is 12.1" and it's just fine. Because of the screen's size I get about 4 - 6 hours battery life without any extra batteries. My buddies with bigger screens get half that and need to lug around a charger or extra batteries on top of their already higher weight.
L.