This is a very noticeable distinction between a professional site and a not-so professional one.
It's the difference between:
yourbiz.squarespace.com (unprofessional) vs. yourbiz.com (professional)
So rather than having solopreneursidekick.squarespace.com, my domain is solopreneursidekick.com
I recommended that you purchase the.com (vs. alternates like .co, .net, .org). I say this simply because most people will assume it is a .com. So if you meet someone at a networking event, it's more likely they'll still be able to find you online (even if they've lost your business card). If the .com isn't available, I recommend the .co