The front is more efficient for sure her but there are times I'd always us the rear ... anywhere that the surface is slippery either due to wet or loose surface as too much braking on the front will cause the wheel to skid underneath you especially if not in a totally straight line.

Also whenever I'm going downhill the rear does a lot of the work for the same reasons .... although sometimes you need the combination of both as the rear's not enough but I'd always balance it so any kind of skid comes from the rear rather than the front so it can easily be controlled / steered through.

Imagine going down a hill full pelt that's got a loose surface and you slam the front brake on, chances are the front wheel's going to go sideways underneath you slamming you to the ground, do the same with the back brake and it'll take longer to stop but you can just steer through the skid and possibly around any obstacle you were trying to avoid.

When I was a kid living in the US, I had one of the bikes that you pedal backwards for the rear brake, it wouldn't be cheap but it'd be a lot safer .... it may be worth looking into combined with one lever for the front brake.

Not a great choice to have to make but good luck with whichever way you decide to go.