Yes, as Mike noticed, we already have a portable Brake Buddy. And, we are ready to move on.
There really seam to be only two choices that meet our needs, Invisibrake or Stay-in-play. And, it is very difficult to choose, both have very high customer ratings.
After viewing many YouTube videos, I eventually found the main difference. Invisibrake is progressive and Stay-in-play is proportional. Progressive begins braking the towed low and increases over 8 seconds (regardless of how much braking the MH does). Proportional is like a trailer brake controller, but only enables when MH brake is on.
SoCal has two scenarios where breaking is challenging and important. In heavy traffic, you sometimes need fast maximum braking, and on long mountain grades you sometimes need light braking for many seconds. I don't think that progressive brakes is ideal for either of these situations. So for my application, I think that proportional is best.
I wish that there were actual test reviews of these systems. I don't think that there is a bad choice, but it is nice to make an informed choice.