E.M.
I am in the same boat as you. I have a 21 month old son and I am pregnant with my second due the end of March. My parents live in NH so when I go home to visit my son sleeps in a twin bed, he has been in it since he was about 18 months. He loves it.
With the next baby on the way I wanted to get him in a twin bed here at home as well. We just got it this week and it is going very well. Like you my son never even tried to get out of his crib. With the twin you will have him getting out of bed on his own because he can. That is what I am facing this week. The second he gets up I am right there and I put him back to bed and say goodnight. I keep repeating it till he stays in bed. I also have the bed rails so he wont fall out.
My son plays in his room even more now that he has a big boy bed. I have a step stoole for him at the foot of the bed so he can get in on his own. He thinks his new bed is a big toy. :)
I don't like toddler beds only because my son is very tall for his age so with it being the same size mattress as a crib he is just too tall. I also feel they are a waste of money. The twin will last you till they are in high school if you want, not just 2 years.