Hello & kudos to you for taking the time to do your research in advance!
Beardies are to iguanas as a Civic is to a Jaguar, IMHO
Sure they're both reptiles, both need UV light, and there's some overlap with types of food. But I think that's about where the similarities end.
My brother has a beardie, and from what I've seen & heard, they are far, far more docile & predictable than igs. Ig personalities all differ, but they can be way more tempermental and unpredictable. And if you're talking about a six-year-old ig, his huge size is going to magnify these traits.
Ig space requirements are also something to consider. You won't be able to keep an ig that age in a tank; he's going to need a specially made enclosure, or a room all to himself. Some folks (myself included) have free-roamers, but I tend to think that those of us who do sort of lucked out in the personality department. Not everyone's ig is going to be as willing to stay off the entertainment center, or not use the couch as a toilet!
I'd check these three resources for detailed information: Anapsid.org, The Iguana Den, and the Green Ig Society.
And feel free to ask lots of questions, we love to help! 