I bought a 1000 hp DSS CF drive shaft from Lethal Performance in December 2017. Cost was $1,169.99 at the time and with a discount code, I got it for $994.49 shipped to me for free. All that to say, try to wait for a vendor sale for a potential discount. Well worth it in my opinion.
Not sure why the choice of one over the other between QA1 and DSS (Drive Shaft Shop). Possibly past experience for those that have upgraded on other Mustangs. I had no previous experience on either.
Summit and Steeda are neck in neck on price as the cheapest, and I know Steeda will run sales from time to time. I can be patient for a sale...maybe...possibly...I think.