LBR's are great fun to shoot, and you won't regret it whichever calibre you get.
There are a few things to consider, firstly, what reloading kit have you got? If you already have a 44 or 38 lever action, its often good to get the same calibre in LBR, so you only have to reload one type of ammo.
Also a lot of people like the 38 (i personally have one of these) since it is a lighter frame and you get an extra shot. However if you have big hands, the 38 might feel a bit small for you.
The best advice i can offer is go into a shop, or down you local range, and handle and shoot all calibre's, 38, 44, 45, 30...
Only you know what gun is best for you. See what you like, and go for it.