6k will hold more detail and give more framing options. You'd think the bigger sensor would yield more dynamic range but I guess that's not the case. 4k 120fps and 1 stop more of dynamic range would have made this camera a lot more compelling. Still close to 3k at 120fps is a decent step up. Bigger sensor is always a plus, if it can be speedboosted you'll get a full frame FOV.
I don't think it will kill URSA sales that much. For someone who likes working with cinema bodies I don't see why you'd step down to a DSLR looking thing. But I think its a great, more options is always better. Glad they aren't incredibly afraid of keeping their "cinema line" safe, as other companies seem to do, other than maybe Panasonic.