Once again, seriously, the 1200S can no doubt be made into a real bass amp. It all depends on what you like.
I own a 300T and I really like it. I think the channel mixing and graphic EQ are BS, but I truly do like the split-band compressor. If you like the mid notch, buy an Eden.
I would love to set a 1200S up for bass. If done correctly, it should sound very much like a Hiwatt 200, but with the Sunn mojo.
The 300T weighs 75LB and is a bit of a pain to cart around. Dunno what a 1200S head weighs. 60LB? Who knows?
In my opinion, the 1200S conversion would sound better; there's not a thing wrong with the way a 300T sounds. Your choice. Rocketboy