Well, I'm not expert; but the Yorks will provide you a better representation of your mix. I just moved from Monitor Ones to the YSM's and the diff. is night and day. The best way to describe the Monitor One's is to say they sound like a nice Hi-Fi speaker. In fact, I used to have a pair of Bose 302's for my stereo; and the Alesis sounded very similar. That isn't nec. bad; but it does have a hyped "low-mid" sound.
For mixing monitors, you want to hear what you've actually got in your mix. The Yorks have really helped my mixes and have some "depth" (Something I had never even heard on my Alesis)
I would go for the York's. Just my 2 cents :=)