In the nutshell the Gradle is much more powerful tool. It left traditional format of a configuration (XML) and took advantages of the closure syntax, so file look much more simple.
In Gradle it is possible to define own tasks and mark dependences - it's memorise me a Makefile. Gradle interprets Groovy language, ex:
from { configurations.compile.
collect
{ it.isDirectory() ? it : zipTree(it) } }
Other advantage is that it can generate test reports and you can separate reports for unit testing and integration testing.
A configuration file is smaller than pom file.