The darker the crystal malt is the deeper the flavor will get. You'll start getting more burnt caramel, plum like notes the darker you go.
Can you post the rest of the recipe? I usually start middle of the road on my crystal malt for darker beers, like the 60* lovibond (not sure what that equals out to in ebc)
Then ill add chocolate or roasted malts for color and roasty flavor.
Chocolate malt for porters and roasted malts for stouts. That's just me tho.