My wife and I did the EB(west)-CS-CZ(east) last year and I did the CZ(west) a month ago. In both cases the scenery on the CZ was very nice. Crossing the Rockies is a wonderful experience. The EB across the plains can be boring but the Sierra Nevada range, Puget Sound, and the CS(extra night) make up for it. There really is no wrong train. I would be happy on either. It's a tough decision, but I vote CZ.
Scenery, CZ - Marias Pass is beautiful, but it is a long day through eastern Montana, which they call Big Sky country because there is not much to see on the ground, whereas the CZ is constant scenery all day leaving Denver, the Front Range, the canyons on the western slope, Ruby Canyon and Book Cliffs, then Donner Pass the next day. You simply can't beat it.
Service, EB. The Builder's diner's service is at the traditional staffing level and wasn't cut back like most other trains a few years ago. As always with Amtrak,onboard service is always YMMV, but the CZ crews generally don't have a great rep. The Builder seems to have more service oriented crews. My own experience is in line with that general perception. Although that is far from a sure thing and your experience on the CZ may well be great and the Builder terrible.