Hi Reuben
I normally steer clear of this type of discussion as the windows vs apple debate can get rather heated.
However, it's Friday evening and I've had a couple of glasses of a nice red and would like to share my experience (please forgive the verbose nature of the following).
I had always fancied a mac, but due to the perceived expense (I built my last few PCs with a mixture of cannibalised parts and new) I couldn't justify the cost.
A couple of years ago I was lucky enough to be given a secondhand iMac. Within 3 months I bought a macbook pro and haven't looked back since. I use my macbook at work and although my transition from windows to longer than it should, I now only use windows for 1 application (through vm fusion).
I find OS X a much more pleasant experience than windows and with a bit of research most of windows software I used could be replaced with open source, free applications.
Anyway, in your situation I would go mac, but consider the following....
If weight is your primary, then it has to be the 'air', my macbook pro 13" weighs in at just over 2kg.
If you need to use windows, then consider whether bootcamp or virtual is the way to go. If virtual (fusion, parallels etc) then you will need to max the RAM (4Gb?) to make the system satisfactorily useable (based on experience with 2009 macbook pro).
The choice is yours
PS the imac is now 5 or 6 years old, still going strong and a useful bit of kit. The longest useful life I've ever had from a 'PC' is 3 to 4 years.