If you are serious about being in US for a while, get the extension on H1B.
Once you move to EAD, if for any reason your 485 petition gets rejected, this is how it goes:
* When you are on EAD, you are considered to be on an immigrant visa.
* While you are within US, you may change your status from a non-immigrant visa (H1, F1 etc) to an immigrant visa (GC) but not vice versa. To change from immigrant to non-immigrant visa status, the only way is to go out of the country and come back in with a new non-immigrant visa by appearing at US consulate in your country.
* If for any reason your 485 gets rejected, you would be in a soup -- in order to continue working in US you need to get a new H1B visa by going outside the country. Since your H1B would not be considered an extension at that time, you would come under the cap restrictions. Based on how soon the 2007 cap for H1Bs was reached, I guess you would end up outside the country for at least 6 months.
Regarding advantages:
* Freedom to Travel -- H1B (You don't need to apply for a separate document, read AP, to re-enter US).
* Change Jobs -- Neither (unless you have your 485 already filed and is more than 180 days old, you may not apply for AC21 while working on EAD also). As with H1B extension, you may change job if your current employer agrees to keep the GC process alive and your new employer needs to file for a new H1B extension based on the GC process with your current employer.
* How long you may extend -- I think you can extend either EAD or H1 as long as there is no final decision on your GC case.