My welding skills are exactly the best in the world so should I try for a butt weld or would it be easier to just go with a lap weld. Pic below.

 

Lap weld is going to be easier, and once you put down carpet and pad, nobody will ever know.

Unless you're using a wire feed, or something you can really regulate and control, why don't you just braze it?

 

I would butt weld that seam and plug weld at the inner rocker panel. For the butt weld, take your time, trim and line up the two panels as you stitch weld it. Grind the bottom of the butt weld, leave the top weld intact and seam seal the top weld before covering with carpet.

 

butt weld with full weld all the way around and then seam seal the seam is the correct method for metal repair. with that being said, a clean minimal lap joint with seam sealer is no different than any of the other lap joints that the entire car is made up of.