The third slot is something you'll either use or not. I use it extensively and wouldn't want to be without it. It allows you a lot of latitude for complex setups, and it always allows you to keep the adjustable tool rest in the bottom slot, and simply pivot it out of the way when not using it, and still use the middle slot for other tooling. I keep the adjustable rest in the bottom usually, and have a few other rests and fixtures I put into the middle when I need them.
The reason the TAG horizontal mode isn't the same as the others, is because back when it and the TW-90 were the only grinders that went into horizontal mode, Chris of Wilmont didn't want to use the same mechanism as the TW-90 and be accused of copying it, even though it's a basic mechanism. Of course, the grinders that have come out since, haven't had this aversion to copying. We can all agree that the horizontal flipping option of the TW-90 is slicker than the TAG-101, and Chris is well aware of it. I think he's been planning a different flipping option for a while now.
FWIW, you'll either *rarely* need to go horizontal, or you'll need a dedicated horizontal, in my opinion. Personally, I cant live without a dedicated horizontal, I use it on every knife I make, but IMO it's mandatory for pocket knives the way I make them.
I had flipping plates on one of my TAG-101s (I've got two TAG-101s and two LB-1000s) here in my shop, but ended up making a dedicated LB-1000 horizontal, and I'm putting together a second one. I moved the flipping plates to a grinder I've got up at my cabin (for versatility since I only have room for one grinder there), as I just never use it anymore on the main grinder.
The way *I* make knives, I'd be better off with 2 horizontals and one vertical, than 2 verticals and one horizontal, regardless of whether the verticals could turn over. I need my horizontal much lower than any grinder that flips, unless I had them entirely too low for use vertically, which is very critical for me when flat grinding.
I personally find it too time consuming and tedious to flip even the TW-90, especially considering the extra fiddle factor of re-orienting tool rests and such, which are much more necessary in horizontal mode (once again, the way I make knives, ymmv).
I've said this before and I'll say it again, in my opinion have the "one grinder that does it all" is only important, if you can only have one grinder. Almost every full timer I know, has 2-3 grinders, in dedicated setups they don't change often, and wishes they had more.
Personally, I'd buy two variable speed LB-1000's before I'd buy any of the other options mentioned.