You only need the one nuance folder. In your case it sounds like the internal memory.
Rename the enu folder that contains the daniel voice to eng - which is correct anyway.
Then copy the enu folder which contains samantha to the same place. So the paths are as follows:
nuance\languages\enu (Samantha voice)
nuance\languages\eng (Daniel voice)
Remove the external nuance folder.
I prefer svox victoria for the simple reason it has an independent volume control with it. I have small speakers built into my crash helmet so when riding my bike I can get that bit of extra volume out of it. It difficult to hear over the top of the noisy hyabusa engine.
Victoria is a paid for app voice. You may overcome this using lucky patcher but I dont know for sure.