I actually went through this process back in 2013.
I loved my 2006 Boxster with only about 60K miles and really didn't have any plans to sell it. BUT, at that time, with all the rumours of the coming flat four turbos and also the possibility that Porsche could be discontinuing manual transmissions, even in their sports cars. Those two factors had me thinking long and hard. What if I did want to trade to a newer Boxster in the future? They could all be missing two of the things that I absolutely loved about my 987. I spent about 9 months wearing out the Porsche configurator. Thinking that in the next few years, I'd want to order a new 981S as a keeper. I tested out the various options when visiting dealers, wanting to get it as close to perfect (for me) as possible. On one of those visits, a relatively new dealership came up with a deal that I decided to accept. Ordered it and 2 months later, I had my new 981S.
For my tastes, there isn't another Porsche - or any other car, that I'd trade my 981S for. Obviously, the 75 HP bump in power from my 987 is quite noticeable. The advancement in chassis development from the 987 to the 981 is quite a step forward. Despite, the fact that I have the Sports Suspension and lower profile tires, the 981S is as comfortable on rough surfaces as my 987 with stock suspension was. The big difference you notice is when you get into the twisties, where my 981 just handles a lot better. I also love the sound of the engine (I opted for PSE to maximize that) and the shifter is more precise and smother that the one in my old 987. It was even better after I changed the shifter for the one from the GT4.
I admit that I haven't driven a 718. I just have no interest in one. The sound reminds me of my old VW Karmann Ghia. It was fine for a car costing $2,995 brand new. I just expect a sports car costing $80K - $100K to sound better.
Then there's the turbo. I've driven both a 991.2, with manual and a 992 with PDK. The turbo lag is there. It's not big/long, but I just don't have it in my 981. I step on the gas and it goes NOW. Not sort of OK, now. I'll just keep my 981S thank you very much. In addition, the turbos tend to muffle the sound somewhat. But it's possible that was because both of the 911's I drove were coupes.
Granted. Soon, you'll be able to get a Cayman GT4 or Boxster Spyder with the 4L NA engine. I'm sure they'll both be fantastic cars. My only problem with the Cayman is the difficulty of lowering the roof.
The Spyder is also a bit more work than with the Boxster. Just personal choice. With living here in Canada, I always drive with the roof down unless it's pouring rain. A few times I've stayed out of the dog house by being able to quickly raise the top while at a traffic light or stop to raise the roof with showers approaching.
Then there are the rumours of another new Boxster GTS with a NA Flat Six (the 992's 3L bored out to 4L minus the trubos) being in the works. The European version is supposed to have a particle filter, but it's possible the North American version won't have this filter. Time will tell.
So, for me, after 57K miles I'm still 99.999% happy with my 981S. (Yes. knocked off that 0.001% because of the "aluminum" coloured tachometer in the S, which is hard to read In some lighting conditions.)
Happy contemplating.