If the lug fits tight around the lug silver is applicable but for loose joints, brass is better from what i've learned.
One benifit of using silver is that it's easy to clean up the lug shoreline; take a small needle file and use it as a scraper to chisel off any excess silver globs. Much easier than removing excess brass since you will have to actually file instead of scrape. Yea, I know, there shouldn't be excess material on the lug edge...but it happens. I'd rather use too much filler rather than not enough, and sometimes it's hard to judge how much is laying on the edge when the whole mess is slathered in flux.