Definitely go for the greater capacity on a V6.
The picture above is for a single turbo feeding both banks of the V6. I don't think there will be enough room in a 155 engine bay for two turbos and their pethora of pipework, besides balancing two turbos is basket diffcult bordering on impossible. Lotus managed it on their V8 by blowing the opposite bank to which the compressor is driven from, but this is mighty complicated. Whatever way, a twin turbo would have to be a one off fabrication.
Autodelta seem to be the way to go.