I've had the original Camelbak Kudu 18 in the past and currently use the EVOC Tail Pro 10 you've mentioned is on your Christmas list. The EVOC protector definitely has a wider and slightly longer coverage than the Cambelbak and also seems to sit more squarely and is held in place better by the pack's better straps. It's less comfortable off the bike as the protector is much more rigid, but it is very comfortable in a riding position and also a lot lighter.
I've had one big off while using the EVOC and it did save me from a potentially more severe injury. The protector (which uses similar material to a helmet and is designed to be a one-hit-only deal) was dented right in the centre, which would have been my spine. EVOC provide free replacements of the protector, which I received within days from the distributer.
So all in all, I thoroughly recommend the EVOC for maximum coverage and I think the more rigid protector is also more likely to help in an over-extension type situation than a fully flexible protector would (although I have nothing to back that up).