I'd say its just about the same. Driving up to Kuala Kedah is quicker than Kuala Perlis, but the ferry from Kuala Perlis reaches Langkawi faster since its much closer.

Speaking of long ferry rides, I'm assuming u mean to avoid seasickness? I'm wondering if there are still ferry services that use catamaran boats. Those twin hulled boats r much more stable in open sea and less prone to causing sea sickness.