From recent user testing sessions at RightsCon 2018:

Issue: Several people were looking for more explanation about Tor, how to use the program with Tor

Suggestion:
Consider creating a separate dialogue box. Since Tor use is seemingly less common that is fine. That would also give more space to explain what Tor is and what the consequences of Tor use are. Proposed text:
"Tor connectivity provides for better anonymity protection*. For more information and to download Tor, go to www.torproject.org.
Connecting to Gridsync over Tor may be slower or less consistent than connecting normally. In order to connect over the Tor network, Tor must be running in the background.
[ ] Check this box to connect over the Tor network."
*we may need to explain more specifically how it would complement the privacy and security offered by S4, in order to not give the impression that connecting without Tor is particularly risky.