What is better for a mustang??
In 1967, the transmission type was not a huge factor in rarity from a production statistics standpoint.
356,271 hardtop coupes were made
44,808 Convertibles were made
71,042 fastbacks were made
If you have a convertible, it is rarer.
In any case, the condition of your Mustang will impact the value much more than the transmission type.
There were two types of automatics in the 67's; a 2-speed and a 3-speed (mainly depending on the engine).
I prefer automatics for every-day driving and performance. I prefer manuals for flat-out fun.
I happen to own a Mustang myself, and I use a modified 4R70W transmission in it. It's pushing around 500hp at the engine, but 442 at the wheels. Automatics have a slightly lower efficiency than manuals, but the shift speed and consistency are un-matched (when properly spec'd of course).