hmmm...I knew it was only a matter of time before this sort of response came out.....
there are just some places it is really really not desirable, or good, to have mat burn. I vote Gi.
On a more serious note. I sweat.. a lot....I like my Karate gi just fine. I can only imagine what wearing a heavy gi would be like considering how much I heat up already. I'm the one who is wanting to crack open the windows when its 19* outside...
In my dojo its personal preference as long as its plain and white. If your sensei has not brought it up you know it is ok to bring it up to him... or to one of the other more senior students who might have the answer for you.