If this is a truck you plan on keeping for awhile and want it to look nice, I wouldn't advise doing either of those paint ideas. All the rattle can paint would have to be removed if you planned on repainting with urethane. Any rattle can is a laquer based paint. You can't paint urethane paint over top of laquer paint. The majority of paints today are urethane.
You need to find out what type of paint it is. If its 87 or new, it is most likely urethane. 86 & older being laquer. If it is laquer I would advise to strip it with airplane stripper(sandblasting tends to cause warping of the body), then DA it with 80, followed with 180, then epoxy primer. If it is urethane base, block it with 180 or 220, then finish it with 320 & prime it. The cheapest and easiest way to get a flat black paint job is with black epoxy primer. You can get a set up at a paint store for around $80. That is what I would reccomend, for a few reasons. Epoxy seals up bare metal, you can do body work over top of expoxy. You can also put a urethane based filler primer over top of it. Hope this was some help to you.