Butt weld vs. flange weld
Admittedly, I've been doing this kind of research as I have been working on my trunk floorpan and planning to redo the seams on the rear quarters. Among welders, it becomes almost a religious debate as to which way is better (almost like a windows vs Apple debate). Both have their benefits and downsides. Mind you, I'm no welder and haven't ever held a MIG in my life (not one that was on, anyway), I can't do anything more than pass on the information I've collected. I expect that to start changing in the next few weeks as I start down the road of learning to weld small stuff.
From what I have been told by various people experienced with welding (I can be a research junkie at times), doing a small flange/overlap requires more prep and cleanup, but it is generally easier to do and it will be stronger. I've been assured by many that any fear of moisture collection at the overlap is unfounded when "done right," which has been described to me as a small 1/2-5/8" overlap (which will sandwich the steel together) combined with sealer, plastic filler, undercoating, etc.
Doing a butt weld isn't as strong, but is generally much harder (requires more patience and experience) to do right and it requires a lot less cleanup and blending. The finished produce won't require as much plastic filler or effort to smooth out the seam.
Again the disclaimer.. I don't know any of the above through first hand experience...
1968 Chevelle Malibu 454, mild cam, 30 over, 12-bolt 3.73 rear
1972 Chevelle Malibu 350, radical cam, 30 over, 10-bolt 3.55 rear
2007 Mustang GT, 4.6L (281), Stock
There are no Ford vs. Chevy wars in my garage. It's all U.S. muscle.