I ended up with several canisters of Spike, a few years back, when Tractor Supply was clearing it out. I tried it on some mesquite trees, with very little success. I wouldn't recommend it, especially at regular prices.
It works GREAT on shin oak, but you're wasting your money on mesquite.
Power Pellets are hexazinone (Velpar). It works great on mesquite and cedar (I've also used liquid Velpar as a soil spot treatment, and it works great). Spike is telbuthiron. Just doesn't work that well (my success was probably 20% or less) on mesquite.