The only reason to grub mesquite is to get rid of the brush in a short time so you can further plow/plant the field.
If you are not going to do this, then, by far, the cheapest approach is to use spray. The time involved and fuel for tractor is vastly less than a grubbing approach. And, if you already have a spray rig, then you also save the cost of purchasing the grubber. If you spray, under optimal conditions, you can get about a 90% kill rate every time you spray. So, make a round, wait a couple of weeks and see which plants you missed and make another round, repeat. The following year, repeat. If you use a foliar spray, you MUST get EVERY branch/set of leaves sprayed. If you miss a branch, the following year all the plant will be dead EXCEPT that branch and its related roots...so just spray it again.
The basal spray method works very well on the size trees you have as long as the bark is smooth...but you must spray ALL around EACH trunk, regardless of size, for about a vertical foot near the ground.
Foliar spray should be done just after the leaves turn from "new" green to the dark green...at this point the roots are replenishing their resources and spray is most easily absorbed and transported to the root system.
I've used both spray systems with excellent results...it is clear that careful and complete application is the secret. Get sloppy, you get erratic results. I have found a hand wand to be the best for spraying...can be precise in placing the spray, and puts expensive spray exactly where you want it...minimum lost spray.
If you, truly, have only a couple hundred plants, the spray approach would definitely be my recommendation, as long as you don't need an immediate clean field. I fear you may have under estimated the actual number.
