This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Since Groovy's syntax is much like Java's it's probably quicker to learn Groovy/Grails than Ruby on Rails since you're mostly familiar with the language the framework is written for.
Then again, you may be one of those people who find it easier to learn Ruby than Groovy just because Ruby looks so different you won't accidentally write Java code when you meant to write Groovy code (though in most cases it won't make any difference).
I hope this helps, but remember Your Mileage May Vary
SCJP, SCJD, SCEA 5 "Any sufficiently analyzed magic is indistinguishable from science!" Agatha Heterodyne (Girl Genius)