I just realized I never posted back here with what I wound up doing tire wise.
My original plan had been to find another set of 18's (the smallest that will fit over the brakes) but that didn't quite happen.
Then I started looking for a set of 19's for my summers, with the plan of using the 18's the car came with for the winters.
I was never successful in finding a set of 19's but I eventually found a set of 20" takeoffs from an XC90. I was wary at first, as that's not a lot of sidewall for our crappy roads around here, but in the end when I saw them I kind of fell in love, so I kept them
I went with 255/35R20 tires for the summers.
I used to use Continental's ExtremeContact DW summer tires on my old S80, and was VERY impressed with their wet performance (you literally cannot make them lose traction when it is warm and wet). On the downside the rubber formulation in them made them a little hard and slippery when it got cold (as expected because they are summer tires). This limited my flexibility around tire changeover time. If October is colder than expected, and I haven't had a chance to change them yet, it gets a little slippery.
So, instead I went with the Continental ExtremeContact DWS06, which is supposed to have the same wet performance, but in a softer rubber compound to make it ab "all season". They have been just as good in the wet as the DW's, and I haven't noticed any real compromises when it is hot. They do offer me a lot more flexibility at changeover time though, which is very welcome. I liked them enough that I put the same tire (in 205/55R16) on my 2000 V70 Wagon.
For the winters I decided to use the 18" wheels that came with the car.
The factory tire size was 245/45R18, but the rule of thumb when it comes to winter tires is that you want them narrower than your summers, to make them better able to dig into the snow. I did some research on rolling diameter sizes to keep things even, while getting a narrower tire. In the end I landed on 225/50R18's. As desired they are narrower, and they are only 0.7% taller rolling radius than stock, which is tiny enough to be a good match, yet also gives a tiny bit more sidewall, and I'll take all the sidewall I can get in the winter here.
I did a ton of research on the winter tires. Swedish car magazines do more in depth winter tire reviews than here, so I got some online subscriptions that allowed me to see the tire reviews. They tend to prefer studded tires over there, but that makes no sense for Boston winters which are usually just cold and wet, and only rarely require driving on actual snow. Of the studless offerings, one tire kept getting recommended highly in repeated reviews, and that was the VikingContact 7.
Continental used to sell two different studless winter tires here. The milder winter Alpine Winter tire, the ContiWinterContact TS (the latest one sold here I think was the TS810) and the harsher winter climate VikingContact series based in part on the technology acquired by Continental when they bought Swedish Gislaved tires. The Alpine winter tires always drove more like an all season, quiet and smooth, but with better cold and snow winter traction. The Nordic winter tires always did better in the ice and snow, but at the cost of noise, vibration and squirrely performance on dry and wet roads.
This used to be the case, but with the latest gen of the VikingContact, the VikingContact 7, Continental felt like they had made enough progress improving the comfort and driveability of the more aggressive Nordic studless winters, that they didn't think it made sense to sell the Alpine oriented TS series here anymore. So I decided to try them in 225/50R18 on my S90. I thoght they were great last winter. Highly recommend them. They were maybe slightly louder than the all season Pirellis the car came with on the highway, but only barely. They were composed both on dry roads and in the snow. Highly recommend them. Just like with the summers, I liked these tires enough that I also put them on my 2000 Wagon (in 185/65R15).
So anyway, the TLDR version is this:
For the S90 I wound up with :
Summers: Continental ExtremeContact DWS06 in 255/35R20
Winters: Continental VikingContact 7 225/50R18
And for my 2000 Wagon I wound up with:
Summers: Continental ExtremeContact DWS06 in 205/55R16
Winters: Continental VikingContact 7 185/65R15
I'm very happy with this combination.
Hope this helps someone else.