I remember both were pretty good. I was able to get some pretty good games in both guns, but you can go through the su-76 pretty quickly so I wouldn't worry about it. Just remember to stay back and let others spot for you because you will die if anything hits you.
Got an IS-3 with the BL-9 this weekend. The answer is yes, you can mount the BL-9 with just the engine upgrade from the IS without any other research.
Downside is you can't mount any additional equipment until the track upgrade. Not a big deal really. Had a few awesome games the first day but yesterday just got into a string of horrible teams that brought down my win percentage considerably.
I'm also starting to see why people HATE lowes. half of these assmonkeys don't have a clue what the hell they're doing in a tier 8 match. It more often than not comes down to which teams has less idiotic lowe drivers, which is sad.