Well if you're diving really often, the DAN insurance would definitely be worth it.
The Master Plan is the perfect cover if you're diving within recreational limits, and it covers the cost for DCS treatment as well. The Preferred and Preferred Plus Plans make more sense only if you're doing deeper than recreational depths, or if you really travel just for diving REALLY often.
The do process everything within a week, though from my experience, they don't respond until you do. I had to email them to ask for my membership number and confirmation of receipt of payment, a week after the money's been credited to them. You can add the additional A$10 for the fast processing fee, which is worth only for really last minute planning.