this all pretty much comes down to 1 question: what do you want to do more?
if you want to play with all the mods, and don't want to miss some of the mods, stay on 1.5
but if you don't mind some mods missing, and really like some of the new features of 1.6, then go for 1.6
there are people who like to keep a world going for months, and there are people who don't mind starting a new world every week.
I'm one of the latter, if I don't like a certain thing in a world, I just end up deleting it and making a new world. by doing that I did find some very efficient ways to start out.
so in my case going to 1.6 would be a wise choice