While the cost of renting the forms may be equal to the cost of the block, it's way easier and faster to set up forms and then blow in a bunch of concrete than it is to lay a block foundation. Plus it takes a lot less experience and skill to get it right. And IMHO, avoid the plywood because it's thin, weak, and takes a ton of cutting and then reinforcing to get it right. For this reason, most masons avoid the plywood and use 2x10s or 2x12s for forming. Plus, it's generally so full of oil and concrete residue that I just can't imagine putting it to any real use, so you'll be filling a dumpster with it. You may be able to sell the 2x forms locally to a contractor, however.
Most times you figure the footing dimensions, dig the trench 3ft deep (or whatever your frost depth is) with a backhoe bucket that is (hopefully) just the right size, then pour the footing to the width of the backhoe bucket (i.e. width of the trench) right to the top of the trench. This costs a lot more in concrete but is way easier, especially on those narrow little footings you're likely to need. Once poured, partially sink a well oiled 2x4 keyway down the center of the footing right where your wall will go. Then pull the 2x4 once the concrete firms up but before it cures. If local code requires rebar to connect footer to wall, it gets a bit more complex. Also, if the ground isn't level and you need to step the footing or if part of the foundation wall has to support some dirt, that also complicates matters.
Once the footing cures, form up the wall over the keyway, add whatever rebar you need and pour the wall, making certain that the top of the wall is level. Then trowel finish the top of the wall and sink your bottom plate anchors.
That's what I would do, anyway. I spent a lot of years as a civil engineer / construction mgr for big residential and commercial builders. This is before I semi-retired (sorta) and went into teaching.
Another thing to think about is this - of all you need is a 8" thick footer, you could just pour it 8" thick then start the wall on that, basically 2' below grade. This saves money in concrete but is a royal pita for the narrow little footings you'll likely need for your garage, because the trench is so narrow you have no room to work when forming. Plus this requires a much taller form, which is where renting forms comes into play (or you're stuck building really tall forms). So money saved in concrete goes to renting forms. This is why most times you will pour to the top of the trench. This way, you only need a short foundation wall, so forming with 2x12's may work for you.
Just my $0.02
Stick frame is the plan, I plan on sheeting it in plywood and wrapping it with house wrap so it's super tight. Building it stick frame just makes life easier when it comes time to insulate, drywall, etc.
