There are a few considerations in assessing value for 2007:
1) Probability that another team is going to sign Turner away for more than the Chargers are willing to match AND pay the 1st and 3rd round picks that likely will be necessary vs. Dunn retiring or being relegated to backup status.
2) If neither of the situations above happen, which player has the most value?
3) If both of the situations above happen, which player has the most value?
For question #1, I think this is a wash. Dunn doesn't look like he is slowing down at all. Thus, I think he will still be the primary guy in Atlanta in 2007. Perhaps Dunn is hit with a big injury or he seriously goes into the tank for the rest of the year. If so, there is a much better chance that Atlanta will go in a different direction next year. I don't see that as very likely. At this point, I would say that there is about a 10-15% chance that another team would sign Turner away. I can't think of too many similar situations in which a RB is signed away as a RFA and installed as a starter.
If neither of the above improbable events happens, Turner has more value. His role in SD is getting bigger as the season progresses and the success of the dual back approach this past game suggests that SD will continue to use it in the future. Turner has fantasy value as a RB3 as a result. Can't say the same for Norwood with Dunn and Vick taking the carries.
If both events happen, I would rather have Turner. He has all the indicia of a full time feature back. Norwood would still be giving up GL duties to Vick or or possibly some big back that Atlanta may bring in to share time at the RB position.
In light of the above, I would take Turner.