Lybrel is a 20 mcg estrogen pill, while Seasonale is a 30 mcg pill. Seasonale has more estrogen, so you will have more endometrial stimulation, so when you do get your period, it will be heavier and crampier compared with a 20 mcg pill. For that reason, Lybrel might be a better choice for you. With a lower-dose pill, though, some people will experience breakthrough bleeding (many won’t, just depends on the person).
You can use any pill in the Seasonale/Lybrel method. Just skip the placebo pills in each pack and take only the active pills. One of the doctors I worked with recommends doing it in this method: take only active pills until you eventually have bleeding (might be less than 3 months, might be longer). Once you start bleeding, stop taking pills for 3 days, then start back up with the active pills where you left off (even if you’re still bleeding). Keep going until the next spontaneous "period." Each time, there should be longer period of time between bleeds, until eventually, they decrease tremendously or even stop. Note: this isn’t dangerous; you just have so little endometrial stimulation that there’s nothing to slough off in a period (or you might just get some of that dark brown sticky stuff, and that’s it).
The lowest dose pill on the market right now are 20-mcg pills. Alesse, Yasmin, and LoEstrin are examples of a traditional 21/7 pill at this dose (21 days active, 7 days placebo). Yaz and LoEstrin Fe 24 are examples of 24/4 pills. A lot of people like the 24/4 pills better, because you take active pills for 24 days instead of 21, and have only 4 days of placebo, so you’ll have shorter and lighter periods after a couple months. (LoEstrin Fe 24 is different from LoEstrin). The 35 and 30 mcg pills tend to be referred to as the "low dose pills" while the 20 mcg pills are "ultra low dose," in case you hear that terminology.
A 10 mcg pill is being studied, but isn’t on the market yet. I’ve also heard, like fizicsGirl said, that the spring-loaded dispenser for Lybrel is stupid, and you can end up discharging pills into your purse accidentally.
I find out on Monday if I’ve matched into an Ob/Gyn residency program, so let’s hope I can put an M.D. behind all of this advice to make it legit! If you have any personal questions, I’m happy to answer them if you PM me, with the caveat that I am not your personal physician 🙂