The Boss 429 is more desireable for $$ factors.
As for potential, I heard a year or two ago a Top Fuel drag race team decided to try using a Boss 429 - based engine rather than a Keith Black hemi. The Boss 429 was quickly made illegal by the sanctioning body. So.. I'd say that with a potential to compete with 10,000 + HP motors in Top Fuel the Boss 429 can be extremely powerful. However, it's another one of those Ford engines.. like the cammers where Ford built to win and the rule makers decided to prevent that from happening. So there is little to no aftermarket and parts are very hard to come by.