Live Mail has 5GB of space. Gmail has just over 7GB. GMail is maxed out at 7352MB. It is not unlimited. You are only gaining 2GB if you move to GMail. If you havent filled in your Live Mail yet, then moving is not going to be worth it.
Plus GMail, while it does have some good spam filters, i still get a lot of junk. I get more junk with my GMail than i do with my Live. I am the exact opposite of Hikari. My Live mail i get only 1 or 2 pieces of spam while my GMail i am going to the web interface almost daily to report 10 or more emails.
Plus with Live you get SkyDrive which is another 5GB of storage of personal stuff, inter connectivity with your live Messenger, Blog, and so on.
While i know Google does have Blogspot, it is still extradited. Live has its own personal blog service not a 3rd party one. Google does have Gears and Docs and stuff, but you can mix your Live account with Office Online.
So each service has its pros and cons. I use both. It is just my Live is much better for me than my GMail.