Depends on what their potential ends up at of course, but by the time you can train a 4/5 to say a MVP cap, you could probably train 2 5/4s to a superstar cap. I'd take the two superstars over the one MVP personally

.
(And yes, I'm aware that a 4 pot could be lower than superstar. but a 4 star ability can have hideous skills, so it balances out

)
Especially if the 4/5 is 19 ;)