Question ~ H1B Extension after 6 years vs. EAD

My current H1B will be expired next year August, I-140 has been approved at 2007 May and I-485 is still pending (June 2004 PD, EB2).

I am starting to prepare the H1B extension and don't know what is exact difference between using H1B (after 6 years) and EAD.

If I-485 is denied while I am in H1B status (after 6 years), can I still work until H1B expired?

Thank you vey much!