I use both gimp and PS. I like gimp better, but some things you gotta do in PS. If there's something you can do in gimp, then you can do it in PS. It's just in a different place.
Tip: If you can't find something, look harder. It's there.
The weirdest difference is, like you said, photoshop's idiotic undo-ing.
- First, go to Edit/Keyboard Shortcuts/Edit/Undo-Redo and change that to ctrl-y.
- Then go to "Step Backwards" and change that to ctrl-z.
- Finally, go to Edit/Preferences/General/Performance/History States and set that to 1000, the max.
About your first question, i think you're talking about the "copy visible" function which takes exactly what you're looking at in the preview and copies it to the clipboard. In PS it's called copy merged.