I've looked a bit at the Instant Tent 6 at other peoples' campsites and it seems pretty weather-tight but not much ventilation. The walls don't "breathe" at all so it could be hard to dry out inside. I haven't yet talked to anyone that's had one in a good rainstorm though. I did have friends with an Elite Weathermaster 6 that had a bit of dripping in the middle of the tent.

My pick in that size would be a Eureka Copper Canyon, either the 10x10 or 13x12. The floor fabric is a little thin, but it seems to handle wet weather reasonably well and has decent ventilation.