At this time (some details are still unknown) I'd say...
- CPS1 12mhz A-board (but can't be a CPS1.5 A-board because its audio section isn't populated and wouldn't work for CPS1 games, only 1.5 games)
- B21 battery based C-board (must be a B21 chip, but it can be battery-less IF you modify it for key operation)
It seems like you WILL need at least basic solder skills to install this on most setups.
IF you are transplanting a B21 chip you are going to need experience with hot air rework and SMD based soldering (yea that shit is HARD/deal-breaker for 90% I'd say).
A big unknown for me (and we are still waiting for DS to say clearly)...
Do you need to use the C-board from a CPS1.5 (+D containing the Q-sound chip) to play CPS1.5 games properly (ie with sound)?
Pretty sure the answer is "yes" but I'm not sure.
If the answer is "yes" then yea... The multi supports CPS1.5 sure, but unless you "molest" an original CPS1.5 you ain't going to be enjoying them.
I don't know too many folks with even one CPS1.5 game, let alone a few so they wouldn't mind committing one to the cause.
Also as mentioned above you wouldn't be able to use the A-board from a CPS1.5, so really you would need the A-board from something like SF2CE AND the C/D-board from something like Punisher.
So now you've sacked two games (one CPS1 and one CPS1.5) to make the multi (that could play them both). :/