Hi minikane, I tried all. Bought Clip Studio Paint in the end. It was called that, but the downloader was on the manga studio site, so yeah, they are that much the same :)
I like inking in all of these packages, but the G-pen in CSP is famously good for a reason. Also that has the most features and that's the closest to the Photoshop interface which I prefer. But all of these softwares evolved a lot over the years so none of them will disappoint you. Honestly they are all great! OpenCanvas lets you record your whole painting process, SAI has a great-great minimalistic interface and all of these programs (japanese) paint engines are far superior than of Photoshop's. So no wrong bet, just personal preference. Try all, choose one!
Also you can try free alternatives like http://medibangpaint.com/en/ or http://firealpaca.com/en
I still have to try medibang, but I already played with the gif-animation feature of Firealpaca. It's fun!
These two and OpenCanvas are somehow interconnected softwares, but in all honesty I don't understand why these genius Japanese scatter their features across multiple apps... But they now better for sure.
I'm on the surface pro 4 also and I'm having the same issue :/ edit: It looks like to solve my issue I just had to update to a more recent version of ClipStudio, which can be done on their website: https://www.clipstudio.net/en/dl
I'm assuming you're using a Mac. I think Inkscape is mostly for Windows/Linux systems (https://inkscape.org/en/release/0.92.3/), but it looks like there is some kind of workaround for Apple-users: https://inkscape.org/en/download/mac-os/
I'm not sure how downloading and installing on Mac works, but it seems pretty hairy. Good luck. :P