I think there are some points to consider about the Sigma Flash.
1. Does it fully support i-ttl mode of the D70
2. Price as compared to the SB600. If it is around the same price as the SB600, then it would not justify buying the Sigma.
3. Color temperature of the flash tube. Some flash tube does not give 5,200K to 5,600K and tend to be a little yellowish.
4. Battery life. Read a review on the Sigma which says that it slurps up batteries.
These are my thoughts, there may be more points for consideration.
Of course, nothing beats the original for full compatibility.