I've never heard a soundfield monitor 1, but based on their web info;
The monitor 1 will be way smaller when you consider it's sub is built in.
The SEOS will be way more dynamic and have true bass with a real subwoofer (or go with Bill's designer series and no sub, probably still better bass than the 8" subs in the monitor 1).
Both should be fairly equal in terms of off axis listening. The SEOS will be controlled directivity and the monitor 1 is a coax, so that's good. It's really size vs performance. The SEOS + subs will be much bigger, but I'd say much better. Again though, there isn't much to go by on the monitor 1. A lot of coax tweeters have a ragged response. KEFs is pretty good, but many are not.