You know, right off, I'm inclined to say Trike! Although the "Reverse Trike" gets my interest and I'm wondering how it feels, handles, etc... The "Street Legal 4-wheeler" might be interesting, but I'd stick with 3 wheels, I think. I know your delima. I have a '79 CB750K that some one had put a "3-wheeler" rear swingarm setup on.
I'll have to get a photo of that thing on here some day. I'm not sure whether to disassemble it and have extra parts OR to see about making it go? I'd feel more comfortable if the rear axle was a bit wider AND the thing actually had rear suspension. So, Personally, I'd go Trike. Just not sure which configuration. Seeing them built in reverse, makes me want to check one out.