Well, nobody on this forum has an RB so the only info we could give you is theory and hearsay. So rather than listening to us guess and fight over which engine is better for this or that, I suggest you contact
unstable-hybrids.com and ask them some questions. They would be the only ones who really know for sure how much more it will weigh and you would get nothing but accurate information.
I say go with the RB, just cauz I want to see one <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>