I'm not an expert equipment operator by any means but some questions do crop up from what I'm reading RE your first post:
How big of an area to dig?
How big of an area do you need to dig for your garage expansion? If it's a sizable area then you might consider stepping up to the larger piece of equipment. There is a risk with the smaller equipment that it simply won't get the job done in time.
Digging Power
Also, the larger equpment will have more weight and thus be able to exert more force on the bucket for digging. This is especially true if you have tough soil to dig through (which it sounds like you do).
Teeth vs. No Teeth
The teeth will make it significantly easier to dig through tough soil. Instead of distributing the force of the buck across a broad edge, that same force is concentrated at the tip of each tooth. It really helps to break up the soil. Like the difference between digging with a pick ax vs. a squared off transfer shovel. I would imagine that you could get things reasonably smoothed down by back dragging with the toothed bucked. Enough that it shouldn' take much more time with a hand rake before hitting the area with a plate compactor.
Stock piling material?
Someone else asked this already but I didn't see a response. Do you have a place to store the material you're removing from the work site? 12" down x garage foot print would end up being a lot of material for a typical garage. Keep in mind that excavated soil is "fluffier" and takes up a greater volume than the soil in the ground. Anyways, you'll need to first put that soil some place and then dispose of it if you can't reuse it around your site.
Same Day or Separate Day Rentals?
10 cubic yards is a lot of material to move around. That's about a full dump truck load. You sure you want to do that with the little walk-behind, especially if you are trying to do the digging and rock spreading in the same day.
Come to think of that, do you think you can dig and spread within the same rental period? Would it make sense to rent for two days, either consecutively or separated by a few days or so?
Bottom line here, not knowing anything else it's sounding like it will make sense to get the larger Bobcat. I know it ***** to have to spend more than you want on a project (I'm running into that with the need to hire a concrete pump truck for my shed foundtation pour), but sometimes it's the wise thing to do.
