Titanium uses it's own methods and translates that code into Obj-C (you can actually see the * incredibly insane* trans-source when you build the app) -- RubyMotion is a direct interface with Cocoa and Obj-C. If you know Obj-C you can immediately start experimenting with RubyMotion --comfortably-- and if you know Ruby and can "read" Obj-C you can immediately pick up the iOS cookbook and get started, like the authors did. Think of it as a dialect, similar to CoffeeScript, where all the same rules apply, but things are just... better. Having zero Obj-C experience and, I admit, no desire to go into the realms of byte-sized memory management, I avoided it for a long time, but after just a few weeks with RM I feel like I have a very strong grasp on both Ruby and Obj-C, and can read and write both interchangeably, including interfacing with Storyboards and Interface Builder. Two languages for the price of one, IMO, and that was well worth the $150 dollars I paid, aside from any products that I may release with the toolchain.

But yeah, in short: there's no comparison between Titanium and RM.