The Preview for Developers does get you fast tracked for updates, but they are Beta and may have bugs that need to be worked out. Also they lack firmware for your individual hardware and are not as efficient. Many reported problems include battery drain, malfunctions with basic apps like camera and other difficulties like WiFi not connecting...
Many love the P4D and I enjoyed it when I was on the program. Then I got a new phone I was taking on a long vacation overseas and I ran the phone without P4D so I was assured of stability while away from home...
If you are a risk taker, get the P4D going. IMHO, the benefits outweigh the risks. If you want tried and true, don't do it.
The updates that Preview users are getting WILL come to your device. All Lumias are getting Windows 10. P4D is not required to get updates. No one on P4D is getting firmware such as Cyan or Denim through the program. Cyan, Denim and other Lumia firmware is a separate update.