I have two of the 300x reels and like them well. Smooth, solid, really instant anti-reverse, don't bind at all under a load.
I got inspired by mine this past weekend and broke out one of the old Mitchell 300s just to see if it was as good as I had remembered. It was fun but I much prefer the new 300x. No comparison.
No experience with the other reel but have heard some nice reports.
Newt Vail