Question Macbook Air 4GB Or Macbook Pro 8GB - 13'inch

I was in the midst of finalizing MBA (4gb only) model available in India with 128gb SSD until my friend suggested me to go for MBP 8gb Ram 128gb SSD , as per him chance of MBA slowing down in 2-3 years time is more and it might loose up the speed drastically. Hence I have started looking for MBP mentioned model.

My usage would be torrent downloads, youtube, internet surfing, some document downloads related to work and photo viewing. i prefer watching movies on TV, hence macbook would be barely used.
Daily usage would be around 8 hours. Storage is not a problem, since I have 2TB WD external HDD.

Could you help me finalize on one of it. Thanks in Advance!

Another doubt I have is - Will the excel, ppt or word prepared and sent on mac will be compatible to use it for windows user. Is Microsoft office available on Mac ? or it is Mac's own software.