When I started I used to do my work in 3 different programs (Sai, MS4 and PS-a pretty old version of it) now I just do everything in CSP. (I had to use gimp at one point because an employer wanted some graphics done when I did not have access to my PS. F*** that piece of shit trash program I am never using that goblin jiz again)
Clip Studio Paint will do pretty much everything you need it to do to make a comic. The only issue I have is that I can't mess with the sound effects as much with rasterizing the layer like I could in photoshop >< This program also goes on sale a lot, to if you keep an eye on the website to catch a sale. Most artists don't need the extra stuff the EX version has, but there are still some that do (like me) so check the version differences on the site to see if EX actually does have things you would use or not within it, if not then just get the cheaper version.
I've heard good things from people using Sai2 though. A few webcomickers I know have been trying it out since they finally put text functions in and it has been working well for them. Lots of very handy new tools.