Monfils who? Seriously, the question should be Gasquet or Tsonga. And in that case I'll say Gasquet, despite the several mental obstacles, he's someone who can last in the top-20 (if not top-10) for several years. I don't see that in Tsonga. He's more of the player who will have a few hot runs every now and then but never really challenge the 'big boys' when it comes down to it.
Tsonga will end up as a much better/more successful player than Monfils.
I think Monfils will be another great junior player who translates into a solid top 30 player but nothing much more than that. Tsonga just has more weapons and with his increasing fitness, he can only get better.