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You can do it either way. Decals go on quick - lay down some masking tape guides for thin stripes, pretty easy to do yourself. Anything wider than one inch or so can be tough - hard to get air pockets out. Best to use soapy water for the big decals. Tricky for the non experienced, myself included!
I think painted ones are the most durable. When you paint them you're not limited to a decal company's product - you can do anything you want. Just mask it off however you want it, (masking tape comes in plenty of different widths) and lightly scuff the area to paint with a red scotch brite pad.