I say body color. I did my 72 Dart like that, and I'll be doing that to my newest project, a 73 Dart Sport 340. On my Dart, I scraped/sanded/etc so the bottom was perfect, then I used POR15. I primed/painted it body color, and I just used acrylic enamel with a little extra hardener. It wouldn't chip, etc, looked awesome, after 12 years showed no signs of rust or wear (granted, I probably only put 10K miles on it, but it sat outside for about 6 years.) I went thru 3 transmissions, drag racing, etc and I barely ever sprayed it off. Well, when the time came to finally clean it (it was F3, light green, and the gunk had made it almost black) I sprayed diluted purple clean degreaser on it, left it sit, took a soft brush and water to it, and it was like new! I'm going to follow the same formula for the Dart Sport. So, to answer your questions, it wasn't any harder to keep clean, and it was extremely durable. And...it adds to the wow factor of your car (Holy **** that thing is clean underneath), you don't get all dirty when you are underneath it, its easier to find any problems, and it, in my opinion, makes it more valuable and sellable. The ONLY thing I didn't like was going thru mud, etc, it sounded like someone throwing rocks at the wheel wells. To solve that, I'm gonna undercoat the wheelwells and any spot that gets hit by debris. Here is a pic of the car before I sold it, after just wiping it off. The paint underneath is 10+ years old.