If you're looking at the Comp 90, I wouldn't expect to see that board for significantly less than retail since it is
brand new.
The quality of the board is good, but weighing in at 180, I'd say that the Comp 95 is a better match due to the higher stiffness. The Comp 90 is also a shorter deck with a pretty high tip angle, so it will feel squirrellier. The nice thing about the 90 is that it has the new T3 tires which are supposed to be super grippy and very durable.
One way or another, either board will be better than the core 8, since they both have nice trucks (Vector trucks on the 90 and full-on Matrix trucks for the 95) and are probably stiffer. So, in the end, if you can swing the 95, you'd be better off, but the 90 will be OK.
Summary:
Comp 90:
Vector trucks: stable, light, but not tunable
Shorter deck + high tip angle = tight turns but possibly squirrelly
softer deck, less pop, more wear on the deck,
may bottom out on hard landings
Comp 95:
Matrix trucks: tunable, burly, but heavy
longer deck => smoother carves, slower spins
stiff deck, poppy, good for your weight
It's hard to do a strict comparison since I can't find all the specs on the Core 8, but chances are, since it's in the Core lineup, it's inferior to either board above.
Brian will have to give the dirt on the GI line, since he's more familiar with it.
All else failing, go to Salt Peaks and stand on these boards. Bounce on them. Jump up and down. Get an idea for yourself. They won't, unfortunately have any demo boards, so you'll just have to have faith in the internet reviews of the boards.
And, yeah. Mountainboards are expensive.
-J