Hello pschyska; I noticed in your post you mentioned bad display quality issues, myL512 has the same problem, enough reason for me never to buy another lenovo.Regarding the mac and dual boot, apparently that can be a problem, there is some info on that in this thread. http://ubuntuforums.org/showthread.php?t=2081105&page=2 I did some checking and found there is very little price difference between the macbook pro and the MBA, so i would probably opt for the pro, larger screen for one thing. If i did dual boot i would use virtual box as suggested in the above thread, another learning curve as i am not familiar with virtual box.
Edit; looking at the specs. for the Dell, looks good, i assume you are going for the 9 cell Batt?
Very interested in how you like it,please let me know, Private message okay.