I would make the trade. Addai looks more explosive than Caddy and Manning is getting more comfortable with Addai, so he will definitely take over as the starter next year, if not by the end of this year.
Going back to last year, after their bye week and this year, Caddy's played 16 games:
Over 100 yards rushing - 4 games
75 - 100 yards rushing - 5 games
Under 50 yards rushing - 9 games
He hasn't shown himself to be a huge factor in the passing game yet. Plus Caddy had a little back trouble earlier in the year (always a concern for an rb). If you weren't planning on keeping Evans anyway, get the higher potential player for the future.
Please see mine.
http://forums.thehuddle.com/index.php?showtopic=184225