That being said I've been using the Rowloff books for about nine or ten years now. If you like the snare drummers toolbox then I think the mallet players toolbox makes sense to use too. I start with it and then go to snare drummers in about November when I start switching back and forth. What I like about it is it doesn't go too slow and let the kids just memorize lines. That can be part of the bad part in a weaker year though too and you end up leaving some kids behind. The lines match up with the Snare Drummers book so you can use the audio tracks from either with both. I've never tried it really, but you could probably have the kids go ahead and be half and half too.
I've looked at the Wessels book and debated about switching to it. I keep looking at it and seeing that the mallet book doesn't make it as far. If I had a full-time percussion instructor on our staff that I could have go work with them more later I think that wouldn't bother me. As it is in our small school program the last time they really get percussion stuff is 6th grade, so I try and get kids to where they can deal with our average HS music by the time they're out of that grade. It lets us use JH kids in our front ensemble at their option.
Just FYI I know this can be debated, but I do all Mallets to begin. I'm careful with grip and we spend time on hand exercises to begin with, but we don't touch the pads until they all get an 80% or better on the 'test of doom' in the mallet book. I get to the page with eighth notes first + 1, and assign a number to match a card in a poker deck. They each get two cards of the 13 possible and we keep going until all of them have a 100%, a 90% two days in a row, or at least an 80% before we touch the pads. Then I go on overdrive on snare for a couple weeks and we begin alternating just in time for the winter concert. After that it's three consecutive days in rotation on each. (I've had some pretty good success with that)