My first thought is that the Roland has more keys than the Yamaha, and as a person raised on piano, that's a plus for me. On the other hand, when I think about arrangers, I think about Yamaha and Korg, both of which have a large number of after market styles that are available for sale. (The quality of the patterns and styles is a matter of personal preference. Can you go play with them in a store?)
So the question really is how you're going to use it. If you're planning on using the arranger features extensively, and you want to bring in new styles and possibly create your own, I'd go with the Yamaha. But if you're primarily a player for whom the arranger features aren't as important, or doesn't mind sticking with the built in stuff, then go with the Roland.
Also, if you're more interested in sequencing as opposed to arranging, it looks like the Roland is a better fit. But sequencing can also be done on a computer, probably with a better interface than any keyboard. So again, it matters how you're planning on using the thing.
But I'm sure that others have a much more in depth analysis they'd want to toss into the pile.
