Your question is not so easy to answer. I’ll make a modest attempt to do so.
First, it partly has to do with personality type. Some folks love coloring by numbers, others like a blank piece of paper. I know I’m simplifying but if you are more of a color-by-numbers person then I would probably suggest staying with Squarespace. It was developed for just this scenario. Over the years it’s added more colors, but bottom line it’s template driven. Very template driven.
Fifteen or ten years ago the same was true of RapidWeaver. Over the years two very interesting developments have taken place that have radically changed what, and more important, how you work with RW. The first change was the development of Stacks. Without stacks you are still in color-by-numbers land. And at that point I’d agree that Squarespace may be the better option. … but Stacks opened up a lot of fluidity in design.
… but then the second shoe dropped. This happened about 5 years ago (I might be a bit wrong on timeline here): the emergence of frameworks. Essentially RW is theme-driven. Frameworks revolutionized things by essentially letting your create your own “theme”. There are now several frameworks out there. There are all very good, but work in different ways. I use Foundry and love it. Others use Foundation and love it. And there’s UIKit, Platform and a few others. However … for a beginner I would suggest using Source. It’s free. There’s a “Super Source” that only costs $25, but the regular Source lets you learn about frameworks in general.
Put differently, Stacks gave you a lot more crayons to use. Frameworks removed the lines or grids from the page so all you had was a blank space to start with. For some this is frightening. For me it’s very relaxing and enjoyable. The bonus is now you can pretty much create any look you want.
The same thing has happened in the WordPress world. Originally it was just for blogs. Then it added standalone pages. But the whole thing was driven by templates. Then about 5 years ago “page builders” (i.e. frameworks) started emerging. And over the last 5 years those page builders have gotten a lot better. In working with my students I have them use Elementor (free) plus Elementor Pro (paid, $50). It’s a great combo and gives you a lot of freedom.
I prefer to use RW and Foundry. But the bigger issue is how much do you enjoy starting with a blank page? How much freedom do you want? And how efficient do you want to be down the road?
… so efficiency is the third key element. With both RW + Stacks + framework of choice I can easily use elements from one website in another. I don’t have to start from scratch. Since I create course websites this is a big freakin deal. I of course do change things from course to course in terms of look and design: but I don’t need to completely start from scratch if I don’t want to. Okay, this sounds a bit like coloring by numbers but the templates I’m using are ones that I created in the past, not something someone else provides me with. It’s a big difference.
As for cost. That’s up to you but if you are trying to make a business out of website development then the issue isn’t really what is cheapest but what will allow you to be both very efficient yet very creative.
There are other factors involved as well, but hopefully this helps a wee bit.