Brian,
It just depends on what your gunna do with the car. If you keep it streetable then the 6 speed would be great but I'm not sure you can power shift it like the old muncies and T10.
The sixth gear overdrive is nice to have. You can run 410 or beter with that deal.
Now if you don't mind not being able to down shift you can't beat the Jerico 4 speed for $3500.00 with linkage but you'll have to cut the heck out the tunnel.
I just sold a Richmond 5 speed that I had face plated for $3000.00 with linkage but again can't down shift.
Tim ran 9.70's with a factory 6 speed in his Camaro. He had to do some mods on it but you could down shift and have overdrive too. It was good enough to run 9's and still drive on the street.
Long story short you have a ton of options just depends how much you'll be driving it on the street. No matter what you do you will have a ball doing it with a stick car. It's best to spend the money up front and do it right the first time. Otherwise you will be great at rebuilding muncie's and T10's.
Give me a call if you need help.