Modern U is just plain fantastic in all the care for detail, in how natural it plays, the sound quality, the closeness/intimacy you can get with it, really as if sitting at a true piano.
Modern U is newer (by years) than Ravenscroft 275 and VI Labs have obviously used that time well to make their piano "engine" quite a bit better still than with the Ravenscroft.
Having said that, the Ravenscroft isn't bad either, it plays really well and a lot of details have been cared for in that one too. But… it's approaching but not entirely hitting the nail as Modern U does. I haven't succeeded in getting it to feel as close as Modern U, although it is a lot closer and more intimate than e.g. Garritan CFX would get.
The sound of the Ravenscroft really divides people. Some really dislike it. Others, particularly in Jazz I think, love it. I like it for some noodling and jazzy improvisation, but in reality almost never start it up. Because when I go back to Modern U, suddenly, I'm at "my piano" again.
This is a somewhat subjective response to your question, hope it is of some use to you!
Cheers and happy decision making,