Don’t have time? Dude. Learning each software is a labor of time and love. XD It’s friendly to you when you cherish it. Anyway, here’s my 2-cents.
MyPaint – Excellent… for painting only! Use this if you’re a fan of canvases and brushes, and perhaps storyboarding, but nothing else. It’s catered for digital painting, but if you wanna use it for comic design then you have no choice but to use another tool along with this one.
GIMP – Jack of All Trades. You can make full comics with GIMP alone, and nothing else needed. But GIMP is an image editor first and foremost, even if artists tend to use it for painting, so it’s great for post-production mostly. Though, it’s like Photoshop, only more awesome.
Krita – It’s basically a great love-child of MyPaint and GIMP in KDE environment. Again, you CAN make full comics with Krita alone. It’s got the painting potentials of MyPaint, and editing features of GIMP. But as with anything, it takes a bit to get a hang of it. Since it’s a great painting tool primarily, it’s actually not as swift as GIMP when it comes to editing in post-production.
Inkscape – Vector graphics, so you can’t exactly do concept art with this, UNLESS there’s a specific thing you’d like to do that only Inkscape can do it. So if you’re looking for vector look, go for it. No questions asked. If you’re curious about what kind of comics Inkscape is capable of, here’s an example: tushantin.deviantart.com/art/Pol … -295704784
Personally, I never bother only learning things when I need them. I learn softwares beforehand, because (in the line of work I’m in, as an animator) I never know WHEN I’ll urgently need them (because bad things happened to me when I wasn’t prepared). Learning interesting softwares beforehand not only made my life easier but also made me enjoy doing creative things that nobody else can even comprehend; at the same time, it brought down my costs of production. So if there’s a tool around, you can bet your buck I know a thing or two about it.
In any case, hope it helps!