Your friend is right- having the load on the bike is much less sweaty!

That doesn't seem like a very big laptop to me, so you should be fine with most panniers. DH and I both use the Transit Pro panniers sold by Performance (http://www.performancebike.com/shop/...tegory_ID=2312). They're nothing special, but the size and price were both right. If I were doing loaded touring, I'd want something better, and I wouldn't mind a easier-on/off retention system. But again, for 3-4 days a week of commuting, they're performing well. They easily hold shoes, clothing (packed in an Eagle Creek PackIt folder), lunch, and a small purse.

I'd probably buy a laptop sleeve for a little added protection and use whatever panniers suit your needs.

Have fun shopping!