That's really up to the individual. I used an 8.5" laptop for over 2 years, and if apple made one today, I'd buy one. The reason the sony is too small is it's more like 5".
I'd check smalldog.com, and watch dealmac.com for mac deals. I'm not sure of a great source for used macs tho. If the 13" macbook isn't too big (I think it's an ok size, carries well, runs well), that'd be my go to recommendation. Enough of a recommendation that I own one, my wife owns one, my brother owns one,my sister has one, my stepmom has a macbook pro, and my other sister has an ibook. All on my recommendations...