Hi Dave, welcome to the Cameralabs forums! I'm glad you asked this question as I was about to pose it on here somewhere anyway!
In the 40D review, you'll see some resolution comparisons against the EOS 5D and a pre-production Sony A700 - both in the 12 mpixel region, and it's clear they resolve greater detail on a lab chart - and I'd expect the D300 to do the same. See this page:
http://www.cameralabs.com/reviews/Canon ... tion.shtml
But does it make much difference in real life? Check this page:
http://www.cameralabs.com/reviews/Canon ... ults.shtml
I'd say there's not a massive difference there... so sure, 12 mpixel will resolve more detail than 10, but will you notice it in real life? Not much.
As for the 51-point AF, this is really for people who want to track subjects which could be moving all over the frame, and may slip inbetween focusing points on, say, 9 or 11-point systems. I'd say that's unlikely to happen often though unless the subject is very small...
So if you're shooting landscapes as you say, the 51-point AF won't offer you much more than a 9 or 11 point system. So it's up to you whether the extra 50% is worth spending for those 2 additional megapixels and the other D300 benefits like the higher resolution screen.
Remember though, unlike the 40D though, the D300 does not have a live histogram in Live View mode and its screen refresh is also 15fps, so may not be as smooth (although we've not tested a final model yet so can't comment).
Given your existing investment in Canon glass and what you've told us about your photography, I reckon the 40D is the way to go for you - unless you fancy waiting for a 5D Mark II!!