I'd go 20vT because it's a more modern engine, easily tunable for real power with little cost once it's installed and it's more than likely to be more reliable and have better fuel consumption. I don't know much about twin 45s, but I do seem to recall that they drink big time when giveing them a bit of stick.
So 20v because it's more reliable, quicker, better MPG. The twin 45s would be seen as doing something away from the norm though as 20vT are fairly common these days, but that's also a good thing as there must be some very good reasons why that is the case.
If you want to do something a bit different, how about a G40 in a mk1,
I think that seems to suit them so well and you dont see many of these done as people normally go G60 or 20vT.
