it's always best to take it down the metal, if you don't there's always a chance of solvent pop when you lay the new coat over old surface.

sandblasting can be tricky, only goes to a shop that does bike frame and make sure the media they use is suitable for the frame material. it's way too easy to use an agressive media to blast it off but at the same time they will take aways material with it. ever try sanding a plastic part with 80 grit sand paper? same concept. aluminium vs steel vs composite material will need different way to stripe the paint. you can get it done by chemical or mechanical way, a good size DA sander will do just fine.

once you take it down the metal, it's very important to use a good metal primer/sealer to ensure the color basecoat consistence. a good example is an epoxy primer/sealer combo, it's water proof/rust proof 2 part primer that also seals the surface for paint. the epoxy base is also flexible enough for offroading, great resistant against chipping, etc. however you can't get it in can.

one word about powder coating, make sure the shop are specialized in bike frame.. there's numerous cases of car rims that been powdercoatd cracks and broke because the temperature in PC process weaken the structural integrity of the rims. go search on google for that.