First off welcome Paddy.
As for cals well that's a hard one to answer for you, I would suggest if you can try both and see what you think in regards to recoil/muzzle blast etc. As for shooting Deer at 800m, takes a lot of practice and skill to pull that off with a good degree of certainty. I would suggest you buy a decent brand of scope with the ability to dial up for those sorts of ranges (any good Euro glass should be up to that sort of thing no worries at all).
Me I don't have the skills to shoot things that far (never tried to learn) so I stick to max 300m or so as I feel comfortable shooting things at those ranges and I honestly haven't seen anything that I need to reach out and shoot that far out anyway.
I went through the same thing as you recently and settled on the 308 because it was a common cal that I knew I could buy ammo for relatively easily if the occasion ever arose (not likely but nice to know it should be doable if I have too) the 300's are pussycats with a break on I've found, haven't tried one without a break but hear they can be a bit hard on the shoulder but no doubt you'd get used to it over time and not like you are shooting heaps in a hunting scenario anyway.
I ended up settling on a Tikka T3x fluted in the end as I wanted something light weight that I could comfortably carry around the hills where it would be used most (mainly for pigs and dogs and maybe deer down the track) I wouldn't recommend getting some heavy thing as more than likely you will be doing some walking after the deer and the heavy stuff gets old fairly quickly unless you are fit and healthy I guess then it may not be such an issue but for a fat bastard like me I need to lighten the load as best I can
and the other reason I chose the light option was there would be more often than not off hand shooting at moving targets so lighter wins that battle every time, you need something that points quickly and easily and the HB's aren't designed for that generally unless they have a short barrel then it's a little more user friendly but still not ideal I don't reckon.
The 308 is fine to shoot in light weight rifles, the 300wm not so much from those who have used them (only used one in a Tikka Varmint config and as I said it was fine with a MB on but not sure how they go without one but sounds like they can belt you a fair bit.
Anyway mate good luck which ever way you go and as I say if you find it's not what you thought it would be you can always sell it and buy something else (preferable to get it right the first time I know) but sometimes we don't and that's just part of life I guess hey.