There is no boost for any particular squad. If all things are equal, a player would get the same development in any squad. If your U18's can handle the U20 league, and you have enough staff to train them at U20, I say go for it!
Last thing to consider, the training calendar pre-sets are different between U18 and U20. U20 has the same presets as the senior team, whereas the U18 has a youth training pre-set. I can't speak to how one preset works over the other, but that's the only tangible difference between U18 and U20 training.