Initially I was very excited by the the rather gnatty looking Canon XC10 but having spent some time analysing the specifications and ergonomics it would seem that I might actually get more bang for the bucks from a Panasonic GH4 + Metabones Speedbooster which would enable me to use all my Canon and other lenses I've acquired over the last 5 years, and also record full 4K.

I think what is putting me off is the inflexibility of the monitor panel, only an upright hinge movement and the lack of a decent viewfinder. I'd love to have a small Cinema EOS camera that records in Canon Log and I have no doubt that the picture quality will be superb but this might now the one I choose on this occasion.

However I'm going to look at the XC10 and C300 Mark 2 early in June and despite its shortcomings I might well change my mind as the thought of going back to a DSLR type camera seems very retrograde, I'd have to start using my pack of ND filters again!

Any thoughts?

it would seem that I might actually get more bang for the bucks from a Panasonic GH4 + Metabones Speedbooster which would enable me to use all my Canon and other lenses I've acquired over the last 5 years, and also record full 4K.

I agree with this. I think the XC10 will be better in that it will be easier to match with Canon cameras, has Clog, and is a self-contained camera system that shoots to the same media the C300 Mark II will use. The autofocus will also be better, which to me is a pretty strong point as I love Dual Pixel AF and if the XC10 is anything like that system, it will make for some great uses of that camera.

On the other hand, it only has a fixed 2.8-5.6 lens on a smaller sensor than the GH4 (and the Speedbooster on the GH4 will make it an even better low-light solution over the XC10), only a single ND filter (you might still need to use one with the XC10 if that single ND doesn't get you where you want), more slow motion options over 60p, higher quality stills (16MP vs 10MP), ability to change lenses if you need something different, and 10-bit out of the HDMI to an external recorder in both 1080 and 4K.

I am personally interested in the XC10, mostly due to the compact solution that the camera provides (I also have a GH4 and will hopefully be picking up a C300 Mark II when it's released), but I want to see how well the autofocus works on it and how it performs at 6400 ISO (what I expect will be the minimum I will need for shooting at 5.6 indoors compared to the C100 Mark II's 850-1600 range I usually use).

One thing to keep in mind is that UHD and DCI 4K are both "full" 4K. One is simply the 16:9 version of 4K and the other is the Academy 1.85:1 version of 4K. They are both the same quality, one is simply wider. Unless you need letterboxing on everything you shoot, you're going to want to shoot UHD anyway.