For a 120g Nosler ballistic tip Nosler lists IMR 4350, H3450, and Reloader 19 as good powder choices. It lists AA3100 as "the most accurate powder tested with this load". I have no experience with that powder though, but I have used both IMR 4350 and Reloader 19 with 140g bullets. Both gave me good results, but I believe Reloader 19 metered better. I'm a bit more partial to it, although IMR 4350 is a classic powder for the 6.5x55 SE.
I personally haven't shot anything lighter than a 140g and just fired my first 160g (Sierra Pro-Hunter w/ Reloader 22) out of my rifle yesterday. It shot so well it was both awesome and disappointing at the same time. I had just found out earlier in the day that the bullet is no longer being produced (it's even gone from Sierras website). Too bad, as that is some of the tightest groups I've been able to achieve in reloading yet, although I am just getting the hang of it...