How broad are his shoulders? As long as they're not super-narrow, I would probably go with the Monterey. It is notorious on here for not fitting smaller booster riders well, but I have had good fits on almost all of my nanny kids, albeit sometimes with the headrest up a click. The key seems to be how broad the child's shoulders are.
OTOH, when I tried out the Parkway, it didn't fit even my larger kids terribly well, and I needed the SG clip to get a good lap belt fit on the then-4yo. There are many here who like it well, though.
If you have them nearby to try out, I would recommend trying them with your DS in the car. If not, I would order from a place with good return shipping. They are both very nice seats (I would ride in a Monterey if I could!
) but neither are one-size-fits-all like, say, the Turbo. The fit is definitely child and car dependent.