It's no different from a full-line carrier vs. a low-cost carrier anywhere in the world. I remember pricing out a Ryanair flight between Latvia and Germany against Lufthansa, and Ryanair was about a third of the cost. On an LCC you usually have a la carte pricing for things like carry-on bags, soft drinks, etc. that are standard on a mainline carrier. Plus Bangkok Airways styles itself as a "boutique" airline which is code for comparatively pricey fares.

The one caution I have is to estimate your baggage and price it out before booking anything. Awhile back I was traveling between Tokyo and Manila. I had a choice between a fifth freedom flight on Delta and one on a Filipino LCC, Cebu Pacific. The Cebu Pacific flight was about 40% of the price of the Delta ticket but when I added in the cost for my two checked bags it would have been MORE expensive than Delta. By contrast, Delta gave me two free checked bags.

But in general I operate on the four hour rule. I am happy to fly with an LCC and do without beverages, IFE or any creature comforts on flights of four hours or less. Anything over four hours (or when a major ocean is crossed) and I will opt for a full-service carrier.

As to your other question, in my experience the immigration queues at Suvarnabhumi versus Don Muang really depend on time of day. I've been through in a matter of minutes. I've also been stuck in slow-moving lines when a bunch of international flights all dumped into the arrivals hall at around the same time.