It really depends on personal preference when the price is so similar.
I would say don't bother with Rebelle 3 unless you have a powerful graphics card. The way the paint interacts with the paper is expensive on your hardware. Assuming you have a decent graphics card (mine is a few years old, but it's decent) then Rebelle 3 is the only software in which paper texture interacts with the paint you lay down. So if you have a watercolour painting background, you'll really appreciate Rebelle 3. Rebelle is hands down the best for watercolour.
Likewise, if you're hoping to learn watercolour, Rebelle 3 is still a great option. It's also very relaxing and intuitive to use. That said, I really dislike doing linework in Rebelle 3. I can't say for sure if it's my computer or if it's the software, but linework is laggy, because even with the linework, the software thinks you're wanting the ink to interact with the paper, which slows it down.
For one project I used Affinity Designer to do the linework, exported the linework as a PNG to Rebelle 3 and then did the colour in Rebelle 3. It was very satisfying to work that way, and freed me up when I needed to be freed up.
I bought Realistic Paint Studio for my kid. I wouldn't use it myself because the export options -- or lack of them -- mean it's not really targeting artists (yet). It's more of a fun bit of software. It's still really great for what it is, especially in comparison to where digital art software was ten years ago.
Paintstorm has been made for professionals but has a brush system which is so different from all the other software which is really cool once you get your head around it, but ultimately I gave up on that software as it was too buggy back when I was using it. Also, since I only used it occasionally, every time I went back I'd forget how the brush system worked. I mean, it's ingenious, and I wish all of the software used that system, but they don't. That guy's update (ie Realistic Paint Studio) seems to be a reaction to the fact that most casual users end up feeling that way about it, but who knows, you might fall in love with it. If you're using it regularly, forgetting how the brushes work won't be a problem for you. And then you'll have a really powerful brush engine on your hands. If you're someone who loves brushes, and tweaking them, check it out.
