Subdomain vs domain

I have this question on two different sites, but will start with the simpler one. I am changing the primary domain name from mypersonalretreat.com to mypurposefulpause.com. I've connected it and all is well. I of course want to keep personalretreat.com and have it redirect to purposefulpause. SquareSpace is asking if I want to make www.mypurposefulpause the primary or just mypurposefulpause the primary.

The real question is around the www. I have another site where I am having all sorts of SEO issues with my https://kimberlysundt.com vs https://www.kimberlysundt.com.

I hope this makes sense. Thank you for any help! 

It doesn't really matter which one you pick, but it's not advisable to change this once you finish getting it set up and it is indexing on GSC.

10 hours ago, KimberlySundt said:

The real question is around the www. I have another site where I am having all sorts of SEO issues with my https://kimberlysundt.com vs https://www.kimberlysundt.com.

What issues are you getting?

Perhaps the easiest thing is to show you a screenshot. This is from my GSC and it is under pages not indexed>Not found (404). I also have quite a few of these in the GSC non-indexed>page with redirect. Thank you! 

I'm not really sure what's going on there, since I can access the ones I've tested I'm not sure why GSC is classifying them as 404, there must be something up with the setup but I'm not sure what. You may have to go through the setup on GSC again.

Ziggy, thank you! Do you know if I set it up again if that will hurt my SEO? And do you mind if I ask one more question? If I search all of these properties (see image), I get different results. I am searching my Primary Domain. Is that right?

30 minutes ago, KimberlySundt said:

I am searching my Primary Domain. Is that right?

Yes, you want to use your primary domain, and the one listed as the "domain property".

I've honestly never had to re-setup GSC, but I don't think it would have a negative impact, it's very useful, but not strictly necessary, Google chooses what content goes online. 

I'm not sure how best to offer any further help, but if you have any specific requests I can try and help.