8 pax... Take the 450...
If you had said 4 pax, that would have been great, each with a pair of seats each so you could stretch out... But, with 8, you will be sat opposite someone with every seat taken for all that time... Not a pleasant experience at all.
A 450 will allow you to relax, maybe sleep, eat and arrive a little refreshed, versus cramped and tetchy. Plus, with the hand luggage and the guitars, you are going to be stepping over stuff too and from the toilet or drinks/snacks at the front. I don't remember the baggage being able to take more than maybe 8-10 large suitcases.