I was in the same seat as you last season. After riding Nytros since -09 it was time to swap for something new, which meant selling my, by then, one year old 2012 153" Nytro.
Like you I had my mind set on the Pro RMK, but couldn't really decide on which engine. I ended up buying the 600 and I couldn't be happier. It currently has about 1500 miles on it with zero issues.
After I bought it I've had the oppurtunity to try an 800 as well and I can honestly say I wasn't impressed. I did expect a lot more from the 800 since the 600 ran so damn great.
The 800 obviously runs a bit stronger although it's not that big of a difference and since I, just like you, ride at sea level I don't think you'll miss having the 8, especially since it doesn't seem to be as reliable as the 6.
Anyway, don't take my word for it, scroll to the bottom of this link and see for yourself (600 Pro RMK at sea level):
http://www.sledtrax.se/articles/1213/members/2013-02-22_12_de/index.shtml
Thanks - just what I was looking for - first hand experience of someone who came off a Nytro. Nytro has to be the best motor - will miss that - if only they would put it in an equally great chassis. Just want something that is fun and doesn't want to fight you every step of the way in stock form.
The 600 seems like a great transition - only wondered if / how much of the low end Nytro grunt would be missed hence the 600 vs 800 dilemma (not worried about top end so much).