Uhhhh........ Oh hell, ride 'em both. I doubt either route scores tops in all categories.
You're gonna start in Oregon? You've got the perfect situation. Ride the TransAmerica to Missoula; stop in at Adventure Cycling and buy all the maps you need. It's cheaper to buy 'em in the store, if you're not a member!!! I've done this a couple times (I truly owe them a membership but I'm broke, too). From Missoula, you can continue on the TA or jog up to the NorthernTier. This might be your best choice, 'cuz the NT is indeed long and lonely between the Cascades and Glacier Park, and you'd skip this part.
I've ridden both of them (mostly) from the West Coast to Montana, and the only criteria on your list that stands out is that the NT feels more remote (fewer towns/stores) in northern WA and ID. When it comes right down to it, I'd hate to have to choose between Glacier Nat'l Park and Yellowstone. Hell, ride 'em both!
OR/WA to Montana, towns/shops/hardware stores can be spread out, you're lucky if you can find bike parts every 120 miles. On either route. But it's not tough or expensive to carry basic parts -- tube, cables, spokes. Start out with a well-overhauled bike.
Beyond Montana, I'd go by
jamawani's suggestion (he knows of what he speaks) and read up on Crazyguyonabike.com . I'd think the TA has more bicycle traffic, it certainly has been around longer, so towns/shops/people would know you're coming (I mean, bike shops, regular camp sites, etc.)
Hwy 101 south to Mexico is pretty good training. Crater Lake, too, especially if you'd ridden UP to it. Don't worry about the climbs, the first week of a cross-country ride is more than enough to get in shape. Slow is OK. So is being exhausted at the end of the day. Only two things about your timeframe to worry about
: yes, there are some stretches where you might have to carry extra water (say 2 extra liters) -- this is where maps help, even state highway maps. The second is to be prepared for cold in the Rockies -- gloves, cap, sweater, etc. It's just good planning; it might not snow in July, but it's not unheard of.
This past summer, I was surprised how much camping fees had gone up -- Nat'l Forests, Nat'l Parks, State Parks (except Oregon, thankfully). If you make use of (free) public lands, you can save a bundle. Or splurge on a peanut butter sandwich
You're bound to run into some of us on the trail, so you'll be part of the flow!
-- Mark