we were there last year at the begining of the halloween parties (and gay days). we weren't planning on going to the halloween party, and avoided the "official " park of the day for crowd reasons.
granted, it was our first time at DL, but we didn't notice huge crowds at all over 5 days. the one thing that is more important L weather.
the winds. if it is too windy, the special halloween fierworks will be cancelled. actually, postponed. it happend when we were there , and the next eveing was our DL day, and since they had the show all set up from the cancelled one the night before, we got to see the halloween fireworks!
so I guess I would wait and see what the weather report is whenI get there and decide based on that.
as a WDW vbeteran who know nothing about DL, the one thing I learned ontheses boards was "see the fireworks as soon as you can. they aren't offered every night and they get cancelled often due to winds."