Scenic is all relative -- and frankly once you get to the Rockies they both hit a 9.5 on a scale of 10.
Consider other factors: wind, weather, heat, availability of services, finding cycling buddies, historical attractions, westward expansion, intermediate stops, accomodating local towns. For a first-time long-distance tour, I'd tip towards the TransAm for all these factors. But that's me, your schedule or priorities might be different.
I've ridden the TransAm, and the Northern Tier from Glacier to Washington. It'd be tough to say which one is "better" for scenery and wide-open spaces. The TransAm -- all the way across -- gets most of the cycling traffic, so you'll meet lots of other cycle tourists; and the local towns are used to it, most towns have city parks/churches/fire houses where you can camp or stay. This can make a big difference if companionship and low cost are important.
East-to-west is my preference, too, as long as you start early (like May 1st or earlier). This cuts down the exposure to hellish summer heat in the East, and gets you over the Rockies at the right time. Generally, the winds on the northern plains (North Dakota, Montana) are less forgiving than in Kansas and eastern Colorado (if you're heading west, anyway).
I'd lean toward the TransAm for a first tour. Better yet, do both
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CrazyGuyOnABike website (CGOAB), the best information is found in the journals of the ones who've ridden 'em.
-- Mark