I've got both Speedcross 3s and the new Mudclaw 300s. They are both great shoes in my opinion and I've used them both a lot over the past few wet and muddy weeks.
The Mudclaws are definitely better in thick mud - no question. You really can feel the difference, especially on slopes. The traction is amazing. So for really muddy routes or wet fells I'd choose the Mudclaws.
But the Speedcross are fine in moderate mud and fine on slopes too (though the insole scrunches up a bit on wet steep downhill). The Speedcross are more cushioned and I'd prefer them if I was out for most of the day. And they are miles better on hard surfaces. You wouldn't want to go very far at all on road in the Mudclaws but in Speedcross the odd mile or two on road is fine.
Fitting wise I think the Innovates are narrower than the Speedcross so I get a half size bigger than what I actually need for the length.