Hello Carlos
It is not about Spock versus Arquillian but instead Spock AND Arquillian as they can both work together.
Spock is not competitor of Arquillian but rather a competitor of
JUnit.
There is a Spock-Arquillian extension that makes the two work together
https://github.com/arquillian/arquillian-testrunner-spock
My advice would be to understand first what Spock does on its own, then what Arquillian offers on its own, and finally use them together.
Enjoy!
Kostis