My recommendation is buy the computer, use it and then ask specific questions. You will not get too much actual help because of the open ended nature of your question.
There are many differences, some better and some not so good. It all boils down to personal preference.
One small thing I find annoying about Internet Explorer is the fact that the menu items are part of the browser window. I find the Mac/Safari approach of having the menu detached from the application's window to be much more convenient.
Personally I find the main difference is OSX is a lot more stable than Windows (XP, Vista and 7). Some software is easier to come by on PC, but if you really need to run a piece of PC software you can't find an equivalent Mac version you can always boot camp.
There are some programs that install things that load in the background but every time I've did a clean install was because I wanted to. Most of the time it was during a new OS release but it wasn't necessary since the update process is pretty clean.
Advantages:
Longer working life for computer without endless upgrades.
Not having to police the computer when using the internet.
Looking after the computer constantly doesn't become part of using a computer.
Disadvantages:
Slightly higher initial cost (but more than made up for with advantages)