1) size: how much are you going to carry? are you camping/cooking, or staying in hotels? pile up all the stuff you are taking, see if it fits. take it to the store, or buy the bags and return them if they are too small. are you carrying front bags too? that said, 1700 sounds pretty small, even if you have front bags too, if you are camping at all.
2) top vs side load: are you going to load stuff on top of the rack/panniers? (sleeping bag, tent) if so, side/panel loading is the way to go, it's such a hassel to have to unload the stuff on top of the rack to get into the panniers during the day. If everything will fit inside, top access is OK, I guess. I have a strong preference for panel loaders, but that's just me.
3) rain: i use lightweight dry-bags and trash compactor bags to organize and waterproof my stuff inside the panniers. Pannier covers don't keep the bike-side of the pannier dry (at least not the ones I've seen) so you would need to do this anyway. The ortliebs are waterproof, but they don't have any pockets.
4) pockets: I like a number of small/medium pockets on the outside of the pannier to organize/hold stuff that you want quick access to. lights, snacks, flat tire supplies, and I carry my fuel container in one of these too, so if it leaks it doesn't contaminate all my other stuff.
5) how many: i really like the way my trek 520 handles with both front and rear bags - much more stable than rear alone.
6) budget: if i had unlimited $, I would check out Arkel Overdesigns. Ortlieb, if you are rich and like waterproof toploaders with no pockets. Jannd and Lone Peak are good quality products, somewhat less $$. I wouldn't go as budget as Performance.
7) Attachment systems... I don't know what to say here, but they have to stay on your bike.
RE: Pacific Coast - COOL! it's gorgeous. Saw your post on bikeforumns... slow down and smell the redwoods. ;> And I second the remommendation for Kirkendall and Spring's book. Also, if you are pressed for time, don't worry about missing anything south of santa barbara, I thought the best parts are all of oregon and california between Legget and San Luis Obispo.
Have a great ride!