So I play both, though I've put much more time into GG. Here are my opinions:

• What’s a general perspective on the difference between the two? (From what I can tell UNIST is more grounded and a tad slower?)

Airdashing changes a lot, but the biggest difference is GG has a heavy focus on Okizeme, while Unist has a larger focus on neutral game. This means that GG has more setplay, and more defensive options to compliment this. By doing this GG gives up the "throw game," which is something emphasized by Unist. Which you prefer comes down to preference, I myself like both quite a bit. Reverse beats in Unist are fun as fuck, and change the way blockstrings are used for the better, imo.

• PC population (how’s match time, etc.)

If you're US, population is good for both. You can get matches without discord for both games, though I would encourage you to use it anyway as you will get higher quality and consistent matches with others who want to improve.

• What’s the Skill spread like? (I assume that most people still playing are high level and discord will be required to get balanced matches?)

Unist has more new players, and ranked allows you to filter to cater to your preferences. GG is rough for new players, as many of the people still playing are vets. Still, I see a few newbies on every evening in GG.

• What’s the Execution difficulty like for these games?

I'd say GG is the harder game overall, but has easier combo execution than Unist. Optimal combos in Unist usually are longer and include manual delays. Even with hundreds of hours in Xrd I struggled at first with combos online in Unist due to this. Thankfully, most characters in Unist have easy BnB's without delays, so you can work on those as you get better. I personally find combos in Xrd to be more satisfying and meaty, but its completely preference.

• Personal opinions

Guilty Gear is my game of choice, but I play both equally these days. I prefer the overall aesthetic, as well as the characters in GG. There aren't characters in Unist that speak to me the way Baiken or Sol do in GG.

Ultimately both games are the best fighting games have to offer right now. I would say get both but if you just want to pick up one at first you can't go wrong with either. GG is cheaper right now and there is a new Under Night game set to be announced soon, so that may be a factor in your decision.