MangaStudio has some great tools that speed up certain comic-specific things, but Photoshop can do all those things too (they're just not as obvious). If you see MS on sale, get it, but I don't think it's worth the full price if you already have PS. Cost aside, it's another learning curve. If you're already familiar with PS, it would probably be easier to learn how to use it better for comics than to learn an entirely new program.
As for layouts: Read "Understanding Comics" by Scott McCloud. That covers the principles that govern layouts, and without understanding those, you can't make good layouts. Time, importance, etc are paramount, and I have yet to see anyone explain these things better (probably because everyone just says "go read
Understanding Comics" instead of making a tutorial about it xP
This article covers some do's and don'ts.
Here's another one, more about visual pacing and speech bubble placement (don't miss part 2 linked at the bottom).
The best thing for me has been to look at comics and analyse what they do, how, why they probably did it, and why it works/doesn't work.
One thing I don't see mentioned often:
Every page should have a focus, and the layout should work around it. You should think of your pages as small units of storytelling. Stories are made of scenes, and scenes are made of pages. Each scene should have a purpose (and any scene that does not should be cut), and so should each page. Just like you wouldn't spill one scene into another (except for effect), you shouldn't have one page spill onto another. What this means is that if you find yourself moving on to the next page because you ran out of room on the current one and no other reason, you're not putting enough thought into the pacing.
Just like the gutters between panels, panel sizes, and chapter ends, page breaks serve as a separator and an indicator of time/space passing. Make use of them.
I highly recommend doing your thumbnailing a scene or a chapter at a time instead of tackling the comic page by page, so that you can keep an eye on your pacing and how your pages work together.