the biggest advantage to wet laid flag is your daily/weekly maintenance. You can blow it off, hose it down, and even pressure wash it as needed. You need to be more careful with a dry laid patio. The biggest down side is that you need to monitor the mortar joints as a failed joint invites water to get under the flagstone and pop or crack it (especially in z5). Also, you may need to do a full on restoration in about a decade. We did one last year (12 year old patio) where we pressure washed all the stone, ground out all the mortar, fixed a few settling issues, and repointed all the joints. Granted, it was a large patio, but it was a full crew there for 9 days.

Wet laid flag also looks more formal. Dry laid flag has (in my opinion) a more relaxed look. It's less expensive than wet laid. In terms of maintenance, yes you have to sweep stone dust in occasionally, and weeds can grow in the joints, but I don't find that particularly onerous. Either type of patio will last decades if built well and maintained.

As far as Gator Dust, I'm 50/50 on it. We used it on the stonework we did for a wedding venue and it works pretty well. It hasn't worked as well at my house, mostly because weeds can still settle into polymeric jointing and if you ignore them for a couple weeks *cough* you end up mangling the whole joint. When I redo my stone walkway I'm switching to regular stone dust.

Mostly, I am echoing what @Revolutionary has said while adding my spin. His comment about dry-laid looking more relaxed is spot on.

Everyone has a different methodology based on their individual experience and what has been successful for them.. Being in Zone 5 will certainly have an impact over the long haul as well. At some point no matter what your contractor does the joints will need to be revisited whether in five years or fifteen years. It is inevitable.

On dry-laid applications we use Gator Dust on about 80% of our applications, though the joints would be too wide under most circumstances with irregular flagstone.

On wet-laid applications we use mortar 90% of the time and have also experimented successfully with acrylic caulk, specifically 3006 White Lightening, on smaller jointed materials. Best of luck with your project. Post some pics when it's finished!

Thank you both very, very much for your input!!

We believe we have settled on the contractor and they are schedule to come this Thursday for a final layout review and discussion on exactly what we want in the patio. They have offered prices for both wet and dry laid. It is up to us as to what we want. I did get the impression that they were not recommending polymeric sand for the irregular stones primarily due to concern that water would get underneath and pop the stones.

Are there any specific questions I should ask? The better the communication between us and the contractor, the better chance we will all be happy at the end.

Will post pictures :)

Thanks again for your responses.

Irregular flag? I would say to talk to the company and ask how they handle smaller pieces. When doing irregular on concrete, piece size isn't an issue. On dust, those smaller pieces are the ones most likely to wobble or shift. That's an instance where I would recommend gator dust. Or, what we do, is we just pick pallets as carefully as we can and discard the smaller pieces. It's a little more expensive as there's more waste per pallet, but it eliminates that worry.