Our fist trip back since the kids were little was 8 days. My husband told me it was too short, so the next trip was 10. For us, that is the perfect number of days. With that said, we tend to stay an average of 8 days. If I was given the opportunity to choose 5or 6, I would choose 6 because there really is so much to see in each park. WE don't like to breeze in and out, hitting just the attractions, but we do not approach Disney like a Six Flags so I guess that is why we take so long. LOL!
I have never been to Universal, my DGD still gets sea sick on a lot of the motion rides, so we are waiting til she is ready to give it a go, but when we do we are planning 3 days at Universal and will stay onsite.
If this is a first trip I really would suggest the 6 day pass so that you can enjoy the parks, and still have a little time to enjoy your resort, drop by DTD, and not leave exhausted.