Thus far I've been quite pleased with ClassicWB Lite on my A1200. I had OS3.9 back when I had an A3000, and it just felt bloated as hell, even on an 040 with 18MB RAM and RTG; ClassicWB is much lighter and zippier.
Yes you are right i got OS 3.9 on my Amiga 4000/30 but it is not very fast on mine, and it eats quite much RAM. I only have 18 mb to start with. it is nice looking i must admit, but i think i need a bigger CPU for it can run better, and some more RAM