Let's not forget that one of the biggest benefits of the 18-day schedule (over the current 18-day Reserve) is that you know your work days by the 15th of the month prior. Preferences are a bonus as they are not guaranteed.
In my opinion, the 18-day is the working man's schedule...and the compensation is accordingly. Be prepared to work 18-days, not any less, because the preferences may not be granted if they result in less than 18 work days in a month (read: no gaming the system). A little thought will tell you that first and last days are not very productive, so the company will have the most efficiency by limiting those low-productivity days (read: fewer short tours).
If you want short tours, go for the 15-day schedule, but at 10% max, it will probably be bid by the senior guys. The rest of those who desire shorter than 7 day tours will need to bid the 18-day and preference 5 and 6 day tours (which you'll know by the 15th of the month prior).
I see the 18-day as a high paying 7&_ schedule...you may work 7 days at a time, and you may not be able to look 6 years down the road and see which days you'll have off, but you'll still be able to make plans 15-45 days in advance, and use PTO when you don't get your preferred days off. After all, the 7&7 isn't that rigid until you find out if you have training by the 15th of the month prior.