I prefer the U under the goalpost mechanic (which is how we do it in our area).
No matter which you choose, there's going to be compromises. I guess I look at it from the perspective of better to cover the more likely play. Yes, you do sacrifice the best look at roughing the snapper. However, that's a fairly rare foul - and the vast majority of the time in HS, you're looking a try vs a long FG, so the U is only a few yards away from where he'd be anyway. The R can help with this too.
With the LJ (for a right handed kicker) under the post, you get marginally better RTS snapper coverage but at the cost of requiring the R to move very hurriedly to the flag if necessary, while completely sacrificing the ability to cover roughing the passer or anything else really goofy in the backfield. The odds of a broken play seem higher than a RTS to me.