Just saw this on Amazon, release date end of May (and at a discount). A bit of a bummer as we just purchased ours last month or so for our upcoming tour in June. Hard to tell how substantial the updates are given the limitations of the "look inside" feature on Amazon. I might spring for a copy anyway given the relatively minor cost compared to what the total spend is for this trip. But I wish RS tours would have let us know when we used our $100!
We went on the Portugal tour last September. Loved it, loved Portugal. While I did rip the tour book apart and bring the relevant section every day I didn't really need it. For dining on your own, you'll either follow along with your new tour buddies and/or go to one of the restaurants recommended by the guides. For free time, you'll likely do the same. I would not bother getting the new edition. If you have an unlocked phone I would recommend getting a SIM card at the airport. Google maps came in handy.
Though the books are updated almost annually, there's no reason to buy a new one that's one year newer. Just go to this website and you'll find any pertinent updates. If you're carrying a smartphone, I would buy one you can use on the device.