I'm not really technical so I can't explain "why" anything. But I will tell you it is better to have more wattage and not need it, than less wattage and need it.
Mostly it will depend on the distances - how big is your studio? If it's small, will you be expanding at any time? Is there a chance you'll shoot on location? Also, if you'll be lighting from behind your bg, your front lights should be strong enough that they're not overpowered, or you'll get a lot more silhouettes than you intend.
I have ProFotos and love them, but don't know that they're "budget". A lot of folks swear by Alien Bees, which are considered "budget" workhorses.