ccroad,
Welcome to the forum. First of all, all three are very good mowers, well made and top notch!
The biggest difference I see right off the bat is that the Ariens is a mower made more for bagging and side discharge while the other two will mulch better. The Snapper, with the use of their Ninja Blade and that Hi-Vac deck, will mulch like nobody's business. With the use of their normal or Hi-Lift blade, it will suck up about anything. The Toro is probably the best of the bunch for bagging while getting small clippings and giving you the ability to mow a lot of area before having to empty the bagger. See, the Toro uses the deck design to give it it's mulching ability so it can use a bagging type blade making you receive bagged mulch, so to speak. The Snapper uses a Ninja blade to mulch with, as I said before, and will mulch the grass finer than the Toro. You'll likely never see the clippings.
Strait axle vs casters? As long as you can remotely engage and disengage the casters, their pretty neat to have. But, if you have to stop and manually do so, forget it, too much of a pain. The caster, when not looked in, make it difficult to achieve strait lines.
Leaf shredder? I don't know, I use the Ninja blade to mulch the leaves. I can either leave them their, put on the bagger and bag up all of the mulched leaf clippings or side discharge them out into a field beside my home.
Aluminum vs steel? This has never been a big deal to me on a walk behind for some reason. The weight of the steel is not a problem because it's not a push mower. Yes, aluminum can be cracked or broken more easily. Steel can rust faster. Pro's and con's to both. I look for the better overall mower for my mowing needs. I would hate to buy a mower that didn't bag well, if I wanted to bag, but bought it because It has a certain type of deck material. Watch what you mow over and the aluminum won't break. Don't let your deck sit with caked grass under it for years at a time and it wont rust.
Stripping rollers, not that I am aware of from the factories. However, there are after market ones for sale and some people on this forum have made their own.
All three are good mowers, and reputable brands. They all have a little different niche, pick what's best for you.
. if you have a NEW lawn then get yourself a low buck special and use it till you have your lawn in tip top shape.