One is supposed to be a darker pink than the other but not a true red. As usual and as you suspected, "Plant tolerates a range of soil pH, however, flower color will vary depending on whether soil is acid, neutral or alkaline." That is from the PW website. Thus, I would expect to have to amend with garden lime.

https://www.provenwinners.com/plants/hydrangea/tuff-stuff-red-mountain-hydrangea-hydrangea-serrata

It might be an interesting experiment to see if planting one in your unamended naturally acidic soil would produce purpley-pink flowers or even blue flowers.

I just picked up a Red Tuff Stuff and was wondering the same thing gardengal. It's going in unamended!

I'd love a report back in a year or two since I've been wondering the same thing. My Tuff Stuff is blue to purple, and ages to a pinkish purple at this time of year.

I did have "Red Stuff" but in last year's drought and heat I lost it :(.

But as many of you know I am enamoured with simple "Tuff Stuff". It was in bloom in mid July and here it is approaching mid November.