This is a hard one. Like you said, it's debatable that DeAngelo will even be on the same team next year (which could be an advantage actually if he's paired with a weaker RB then Stewart) and he is older and has had an injury history.
On the other hand though, annointing Matthews as the next LT is risky as well. We haven't seen him on the field yet. I'm sure everyone who took Forte or Slaton last year in the top 5 picks will tell you that buying into hype doesn't always get you solid results.
I had DeAngelo last year and although I was always worried about Stewart, DW always produced too. He had only 16 yds and 3 TDs less then Stewart because he missed 3 games. Both had 1,000 yd. seasons. Williams had four games with over 100 rushing yards (over 120 rush yards, actually), seven games with 100 total yards and eight games with 100 total yards and/or a touchdown (all came before his ankle injury).
I would take Williams this year and play it safe. If your pick was lower in the rd. then I'd take Matthews vs. a vet in a RBBC situation.
Help with Mine?http://www.fantasyfootballcafe.com/forums/viewtopic.php?t=468575