1. We've never seen the Borg _try_ to assimilate any of the Dominion species, so there's no evidence that they can't. In one of the DS9 novels the Borg fail to assimilate a Founder, but that's the only example to date that I'm aware of.
Changelings have no blood. Vorta are immune to poison and foreign objects... Jem'Hadar the same. That was stated on screen.
2. Since when do Dominion ships jam sensors? Please cite an episode or preferably a few episodes to support this claim.
All fleet battles --> "Sir, the Dominion are jamming our sensors.. Comm system is jammed... countermeasures Mr. Nog... trying Captain Waters" See: Sacrifice of Angels, Valiant, Call to Arms
3. We all know there's more to a ship than size...even if there was a Dominion ship bigger than a Borg ship that wouldn't mean anything in terms of power. The Krenim timeship was arguably the most powerful ship in all of Trek...if it fired at the Borg homeworld all of the Borg problems would be solved in one swift stroke.
That is true, I'll give you that one. Hell, Species 8472 ships were uber powerful and they were not much bigger than Voyager.
4. Dominion phased polaron beams overwhelmed the Odyssey, but later shield designs were shown to withstand them.
Yeah, because the Federation captured the Jem'Hadar ship. The Borg have no information for them to adapt...
5. Please cite examples proving that the Dominion can build ships faster than the Borg. In First Contact a Borg cube launches a sphere that, for all we know, didn't exist until 5 minutes prior to launch.
Borg vessels are enormous. Yes, this is an assumption... but the Jem'Hadar build vessels faster than all the Alpha Quadrant species and Borg vessels are dozens of times larger.
6. I would argue that Hawk's assimilation in First Contact occurred significantly faster than the Jem'hadar maturing in "The Abandoned."
We're going head to head here. In an exclusive battle between the two, Borg babies in their maturation chambers don't grow as fast as Jem'Hadar clones, which go from infancy to maturity in 3 days. One, from "Drone" became mature in about a day and was described as growing 25 times faster than normal (as stated in Memory Alpha).
7. Shelby speculated that a Borg cube would remain functional even if, what, 70%+ of it was destroyed? I doubt 10 attack fighters, even ramming a Borg cube, could inflict that much damage. And even if they could it's likely Borg shields could adapt to prevent the Jem'hadar from succeeding in ramming them.
That is speculation. And since when do shields protect large ship-like objects from ramming the hull? They did it the entire Dominion war against Federation and Klingon capital ships.
Honestly, I'm not sure where you're getting the information for a lot of your allegations.
I'm favoring the Dominion for those reasons.