I'm also a SAPUI5 beginner coming from the ABAP world.
I've compared now both sides, deployment to ABAP and Java stack.
And to me, the deployment to ABAP seems much more sophisticated than to a Java stack:
You can deploy very easily from standard Eclipse environment (right-click on "Team->Share project"), you can use STMS transporting system and you have the source code uploaded to the server.
If you go for Java instead it's much more complicated: You have to use NWDS, build web modules for your SAPUI5 application, take care of the versioning etc.
To me it really looks that SAP is putting more effort into the integration of SAPUI5 into the ABAP than the Java stack.