TST 507 system with internal sensors on the fiver and external on the truck rears. Batteries on the internals are supposed to last 7 yrs. I had one die prematurely, TST replaced it w/o question. Not that big a deal to remove a single tire for replacement. But the tricky part will be to try managing tire replacement and sensor replacement at the same time on 4 wheels in say another 5 years.
Internal sensors are definitely more accurate on temps and more importantly better able to detect high temps due to dragging brakes or dry bearings in addition to tire issues.
Totally happy with the 507 and TST support. While a tpms will not detect all types of tire problems quick enough to prevent failure, it does help reduce risk of major damage. I did have one failure that it did detect and allowed me to stop w/o collateral damage.