I would think the Master's 4/5 would have less people, and maybe be a safer race. Whereas Cat 4/5, you have a wide mix of people and skills often. Not that masters don't. But down in FL, you rarely see Master 4/5 races. It is either the standard categories, or it is a Masters 30,40,50 etc...field.
I've seen people smashing/piling up in every category, so I wouldn't necessarily say either is less dangerous/safer. But, the few people, usually means more safe, maybe the Master's fields would offer that up as a better first race?
Or maybe this is all crap and do what you want?