Lots of Crystal 20 or a little of crystal 120
 

What is the difference between adding something like 2 # of crystal 20 or 1/4 # of crystal 120 to a beer. I know that the amounts can be changed so that the color is the same but will there be any difference in sweetness or complexity?

There's a huge difference. The Wiki has descriptions of the various crystals. Basically, 20L will be more of a candy sweetness, the 120L will be more raisiny.

thanks! That's actually a very helpfull link. Here i thought it was different strenths of the same malt but really it's all different malts that are similar.

I keep seeing people saying that, but I wonder what the threshold is. I recently used over 10% in a porter (maybe more of an English Dark Ale), and I don't taste any raisin flavors. It did taste raisiny when it was young (1 week in bottle), but after 3 weeks in bottles, the raisin flavor disappeared and the beer is quite good.

You say that like raisin is a bad flavor. I liken it to carry more burnt, molassesy, and rich flavors than lighter crystals. I think the use of adjacent lovibond crystals is more about color impact, choosing 40L over 20L for instance, but once you get at least 40L apart, there's a definite flavor impact in quantities over say 5%.

I didn't mean to imply that. I actually thought it was an "interesting" flavor. When I first tasted it, I thought... wow that tastes like raisins or prunes. But it was interesting that the flavor literally disappeared after a couple more weeks in the bottle.

I think just about all flavors mellow/meld. I just had that happen on a smoked porter where smoke was prominent for about 2 weeks, now it's subtle.

Besides of color and taste by adding more of a light colored crystal you increase amount of non fermentable sugars hence impacting FG and body of your beer.