A lap weld is easier to do and more forgiving especially when your cut line is not perfectly matched. Personally I like a butt just because when your done it can be impossible to tell a repair was made particularily if you grind both sides. I Tig weld all body pieces so a butt weld is a piece of cake, kinda like doing a zipper up...almost no filler wire required if you do it right. No doubt a butt weld is more work but if perfection is what you seek it is the best looking and more durable from an environmental exposure point of view. To make it easy drill holes for Cleco fasteners and back the weld with a thin copper strip full length (using the Cleco's to hold it all together) to help avoid distortion, a wet rag works too.
BTW I am certified for SMAW all positions and as of last thursday also GTAW and GMAW in stainless, steel, aluminum and also titanium up to 1/8 inch. Yea! I passed.