As I've stated in other threads and from experience, you'll get little exp taking 1 char through instances or killing mobs with a 70, even in the lowbie tags it first. I dunno why but a single char seems to benefit not at all from this help; 2 or more chars however turns the exp tide in your favor and makes it worthwhile.

I'd go with the lvling duo and run legit 5man instances if you can; it'll be overwhelming at first trying to control yourself with others but it's good experience and best to get done early on rather than later. While soloing you can do more than asingle char can too, may get less exp per kill butyou'd be killing twice as fast to make up for it, and with less downtime/worry.