What Andy and Clyde said...
SIGNIFICANT difference in stability between the two. The Fenn Mill is a quite 'caffeinated' ski. It's still one of the quickest for out and out speed, but it's fairly 'old school' at this point. The new Elite, etc. offer the same speed with better stability.
If you're supremely comfortable in the XT, the Mill will be a challenge, but doable, if you're inspired, at least on the flat. The main difference is, most can probably jump right in the XT and paddle away, at least without falling in right away-in the Millenium, most would take a swim pretty quickly.
1 Canoe/Kayak Trailer
Bent Shaft Canoe Paddles
URCHIN Portable Anchor