Just looked it up in the wiki...i suggest you go there to read up on everything. This is what the differances are...
10L
This light Crystal malt will lend body and mouth feel with a minimum of color, but with a light caramel sweetness.
20L
This Crystal malt will provide a golden color and a sweet, mild caramel flavor.
I have no experience to tell you specifics, but it looks like they would be very subtle.
Loop