[olddog]

EFI vs Carb:
Many tuners today have never seen or worked with a carb and that trend will continue.
Most of us older guys cannot or do not have the tools to tune EFI.
I have seen quite a few Cobra's launch and slosh the fuel away from the jets and cause a major stumble, likewise in a hard corner. This is solvable, but my point a Cobra may give extra challenge to a Carb.
EFI is much less likely to have vapor lock due to the higher fuel pressure and no fuel boiling in the carb.
Either can be made to work. If these cars are well sorted out it may not be an issue. Ask yourself which one you would rather work with if there is a problem.

5.0 block verses 351 W block:
The Windsor is a much stronger block at the expense of weight. The Dart block a big plus in my mind. The 5.0 block has been known to crack from the main web up into the cam bearing at high HP levels. In theory the less weight of the 5.0 would handle better in an Auto-cross, but car set up and balance between cars is a bigger issue. I wouldn't let the weight issue sway me very much.

I would be more interested in the internal parts in these engines.