The first was do you prefer let's play commentary with only a single person or with a group of people and why?

I prefer multi-person Let's Plays. But there's only, like, 2 channels that I regularly watch. Why? They can bounce off of each other. A single person being funny/interesting for 12+ minutes isn't easy. With multiple people, at least there's some conversation.

The second was is it worth trying to compete on Triple AAA or Brand New titles vs playing something older?

All 3 options suck, but for different reasons.

Triple A games have everyone playing them because they're well known and sell tons of copies. Guy X was going to play through Recent AAA Game anyways, why not do a Let's Play of it? That's the mindset of a lot of people (probably).

Brand New titles, i'm assuming you mean small indie games, are hit or miss. There's so many indie games that come out, sell a small amount of copies, and just fade away. The less people who know about the game, the less people are going to be searching for it. Also, there are a lot of people who sit on sites like itch.io and Steam Greenlight looking for small games that they think have potential to get popular. Indie Games isn't a niche anymore.

Older games simply generate less traffic than newer games because they are old games, so the interest for them fades away. Older games aren't good for growing your channel. But if you want to put out content for your subscribers that you know watch every video, like the 200,000-ish people who watch every Game Grumps video, they are good to make. I look at them as "filler" content.

I like group commentary but someone pointed out that that can lead to in jokes. But this can also help build a community and they like to be "in" on the joke. Perhaps I should mix it up and do some solo videos.

This one I can't give a good answer on. I'd probably try to stick with whatever type of videos you like making the most.

do you think commentary should be revolved around the game and gameplay and whats going on OR keep the banter open and be about whatever is interesting

Try both. Don't force yourselves to just talk about the game, because that can get boring. On the other hand, some of the funniest and most interesting things that i've seen/heard from Let's Play videos comes from discussions that have nothing to do with the video. Keep this in mind, your subscribers aren't subscribing to you just to see what games your playing, they're subscribing to hear you.