Aftermarket hands down. The factory stuff has no manufacturer support anymore so you are stuck with people with good intentions on the inter web for your support and very little of actual trained professionals. Ive heard the complaint of what about parts availability on the trail... well cupcakes, most of the 90 to 96 mustang stuff is usually a no longer stocked item along with the 96 to 2000 explorer setup. With units like my favorite, FiTech, I can get all of the sensors in town, coolant, O2, TPS etc as they are all common in production GM parts... as much as it pains me to mention GM here but in reality this is your best available option in that regard. The only thing they won't have available is the ECU which goes the same for even brand new cars off the lot, you're stuck ordering that and waiting a day or two however I have yet to see an ECU fail so while it is possible, it will be very rare.