Having recently stopped in both places, and under the circumstances you describe, I would recommend Santa Barbara. While SLO is a nice enough town (if not in SB's league, aesthetically), aside from one expensive downtown option, all the hotels/motels require a cab ride from the station. And your Surfliner ride along the coast next morning is likely to be under overcast skies (the sun doesn't usually come through in this part of California until late morning) - and may be fogbound (which was my experience). In any case, I regretted having left the southbound CS at 3:00 pm on a beautiful day in order to overnight in SLO.
In Santa Barbara, there are a number of B&Bs/inns within a short walk of the station (toward the ocean). I chose the medium-range, but very nice Hotel Santa Barbara, a quarter-mile up State St. - walkable, even with a suitcase in tow.
[And while I would not describe the prospect of overnighting in LA (anywhere near Union Station) as "horrible" - and having had the misfortune of growing up there myself - I can certainly understand why you are looking for an alternative.]