Seismically the the routes are a wash. Both cross the range-front ("White Wolf") fault system on the north side of the Tehachapi's - pretty much continuously active with micro-quakes, and occasional bigger ones (3's and 4's are common; biggest was a pair of 7+ in 1952 that pretty much flattened Bakersfield and broke up the SP Tehachapi line pretty well). Both cross the San Andreas. Both funnel through Santa Clarita into the San Fernando Valley (where the 5/14 interchange had the same ramp flattened in 1971 and 1994). The Tehachapi run also crosses the Garlock Fault near Mojave, but that one isn't as active (at least historically) as the others - fairly common microquakes but seldom anything big enough to even feel let alone do damage. So in either case the engineers have their hands full designing a system that can handle earth movement.

The Tehachapi route is probably a little easier to deal with from a seismic and geologic standpoint. Fewer big tunnels, and the fault locations are pretty well known and can be crossed on the surface rather than in a tunnel or on a bridge or very high fill (easier to fix when, not if, they get broken). On the other hand, there's more work on somewhat unstable hills (how often does the UP/BNSF line, or Hwy 58, have to pick up rockslides?). Of course, it's longer and has more curves, so if the object is minimum time to LA that's a problem. But it lets you have a station to pick up High Desert traffic in Lancaster or Palmdale (possibly helping LA's Palmdale Airport - perennially unsuccessful at attracting traffic because most people Can't Get There From Here on the ground) which is a plus economically, and would be easier to hook up with the Las Vegas run with a branch to Victorville (no need to cross the mountains twice). Also, while the Tehachapi route might have some 3-4% grades (probably not very long ones), Grapevine's would be 4%+ and longer requiring more power.

On balance, I can see why they would use Tehachapi even though it's longer. The cost is more predictable since you can mostly see what kind of geology you're running into, the seismic issues are a bit easier to deal with, and you get the extra $$ from an Antelope Valley station (which can always be skipped by expresses - you don't HAVE to stop just because there's a station). Against that, it's longer and less straight so the schedules would be somewhat longer and less reliable. Oh yes, and it snows in Tehachapi a couple of times a winter; less of a problem on the Grapevine route because most of it's underground. Don't you just love balancing acts where there are goods and bads with all the choices, and sometimes you just have to settle for less bad? Overall the issue was fairly well covered in the EIRs published so far. ;-)