In my experience, it would be better to purchase with a credit card, then do a balance transfer to another card that has a promotional low rate or 0% for balance transfers. The interest rate on their payment plans are kind of high from what I’ve heard.
My understanding of the differences on the Plus and Pro are, first, the Pro is designated a class 4 because of the passthrough slot which I found here: Email update June 2017: Glowforge shipments underway, schedule update, and more
The Glowforge Basic is a Class 1 laser - the same classification as a DVD player. It’s fully enclosed and automatically shuts off as soon as you lift the lid.The Pro, however, is classified as a Class 4 laser because, with the passthrough slots, it is not completely sealed.
Then the other official differences are listed here, which mentions that the Plus is a Class 1 on account of being within a sealed machine, but has the same capabilities of the Pro’s laser: Hardware | Glowforge
I’d agree that keeping your plus and adding a second machine to cut your labor hours in half is a great idea; it’s something that I’ve been contemplating for nearly a year. You may not need the Pro’s passthrough, but it’s not a bad idea to have it available in case a project comes up that does require it.
Hope you’re able to figure out something that works for you!