I prefer PayPal only out of familiarity. I've been using PayPal for invoicing for years. In practice I'll take any form of payment that doesn't cost me much more than PayPal (percentage wise). I recently installed the Cash app for a friend to pay me for some hardware I'd ordered on his behalf. I wound up getting their debit card as another tool to protect my real debit/account numbers when making purchases. Fees seem reasonable even cheaper than PayPal in some respects.
The only big difference I've noticed with Square is the ability for customers to tip. I'm not sure exactly what you're invoicing with PayPal - but that's part of why I use Square. Also, the branding on Square invoices is better I think. PayPal keeps their brand front and center, where Square has a bit more of a customer-brand-centric invoice look.