Insulate an attic or a crawl space? Which one would be the most beneficial?
Ummm...I'm going to resist telling any jokes about "feeling the cold air" in LA and get to my answer. 

Add insulation to your attic, if there are more "heating degree days" than "cooling degree days" each year. 

Now, there's one exception to that: You mentioned you have a "forced air heater". You did not mention that you have air conditioning. So, if you don't have air conditioning, then "cooling degree days" don't matter...and you should add insulation to the attic. 

Add insulation to your crawl space, if there are more "cooling degree days than heating degree days" each year. 

If all that isn't complicated enough, you could just put a parka on when the temperature drops way down to 68 degrees or take the parka off when it jumps up to 74 degrees. (Sorry...I tried to resist.)

However, for me, I'd add insulation in the attic, because: 1) on cold days it will reduce heat loss, and 2) on hot days it will keep the hot outside air from over heating your attic and then radiating into your house.