Flanging is typically easier, but as TA said, potentially problematic. If you flange, make sure to lay weld thru primer on each mating surface, then make sure to seal the flange. Some guys will weld the entire seam, but note that you essentially have two seams on a flange weld, and one of those seams is typically hard to get to.
Butt welds can crack or flat out break. They can be tough to master. Panel fit is a must. Check out my threads on quarter panel install. If you can replicate that basic process for floors and stuff, you'll do OK. Take your time and use air to cool the metal between welds. Use a soft wire that is easy to grind. I use "Twenty Gauge" brand wire from J. Harris. Use flap discs and 3-M rolocs to grind the welds. Grind both sides of the weld smooth and flat.