By the way, if you do decide you'll go the Air route, I would be remiss (again, noting the storage limitations of the SSDs) to not mention to you the software product
Xslimmer -
Xslimmer - Your Mac, on a Diet!
It significantly reduces the size of software - built-in and later added by you - and could, it seems, help in the start-up time of software, too. Basically, it takes a look at your language and hardware settings and removes not only the unneeded languages (which freeware like
Monolingual can do), which takes a significant amount of room in software, but it also strips out any unnecessary legacy (pre-Intel) code and other flotsam.
I've run my Air with this software almost from the start and the first pass with it reclaimed tons of storage from the software, while I run it occasionally to scan through new or updated software - almost always finding stuff to clear. I've never had an issue with any installed software because of this process. You can choose what to scan or remove, but otherwise the product's Genie has been fast and accurate.
Absolutely the best $15 I've spent on software for the Air. There's also a free trial version, just to give you a flavour for what it can do.