Because my son HATED the pack & play & we traveled quite a bit, I decided (after the ped's recommendation) to move him into a TWIN size bed (not a toddler bed) at 18 mths. I would've felt fine doing it even earlier after I realized how dang easy it was & how well he slept on ANY real bed after that!!
Earlier is easier - that's what our ped said & I'm a believer. If you got with a toddler bed, it's just another transition you'll have to deal with down the road (just a thought). I bought the longest bed rail I could find, turned the rocking chair around so the back faced the end of the bed & it was just like a crib at that point. I put a small pillow at the small "opening" at the head of the bed where the bed rail started. DS cried for about 2 seconds & promptly fell asleep - slept through the night his first night & ever since then. It's been WONDERFUL.
Just keep the baby monitor on the first few weeks to make sure that if he IS getting out of the bed, you can be there to help him out. After some other recommendations online, I put in a fairly bright nightlight for that reason - if your child does get out of the bed, you don't want him to hurt himself simply because it's too dark. My kid is a MAJOR explorer, & I thought for sure I'd have an issue with him constantly getting out of bed & wanting to get out of his room. But I think the "earlier" part of the process WAS better for his personality type because he didn't quite "understand" yet that he had some extra freedom. Make sense?
Good luck - I kept the crib up in the same room, thinking it would take awhile to get him used to it, but after a week, we took the crib down & he was good to go!!