Well, the first question is what do you use your computer for, which applications and for what purpose? It is practical (need) vs. want - love cool new toys.... Most apple people I know is the latter, unless they use it for graphic design work -
Edited to add, but many, including myself, love the simplicity and beauty of a Mac. It just makes sense to me more so than PC. No viruses either...
Do you just surf the web, do email, and have your music and photos on your computer? Do you have an iPhone or iPad and want to sync with Apple products in the iCloud? Computing is very much in the cloud these days and will become even bigger. I think Apple is well ahead of Microsoft in that area, so I'd vote for going with a Mac. Either Mac OS X (Lion or Mountain Lion) are very good, but as Fishy said, Mountain Lion has some cool features and it makes it is easier to work iCloud. Safari has some cool features too.
I use a PC at work but also use my own Macbook Pro for Adobe Illustrator InDesign, etc. I go back and forth using both computers for work. I have Office for Mac '08, which I should upgrade to 2011, but it got bad reviews so I may wait a while. I do Microsoft Outlook for mail and calendar on my Mac (sync in icloud for iphone and iPad for calendar and contacts- and photo stream is cool).
Since i have an older version of Office on my Mac, and work has a newer version, there are some formatting issues at times, but 90%, things are okay. Also, even though I have Office, I also use iWork software on my Mac. I like Pages alot (better templates), and save it as Word. I do the same with KeyNote (saves as Powerpoint). Financial stuff and spreadsheets, I use Excel more than Numbers.
A little side note, my mom used PCs for 20 plus years and
hated the idea of switching-- until I got her an in iPad! After she had the iPad for a while, I gave her my older Macbook, and she recently bought an iPhone. She is 77 yr old and has it figured out (with my help and some classes at Apple). The
one to one program is awesome - if you buy a new Mac, you can purchase the program ( $99 ) which gives you one on one training (1 hour sessions for 1 yr). They have free classes too.
http://store.apple.com/us/browse/campaigns/onetoone
if you are totally new to Mac, watch this video on switching PC to Mac
http://support.apple.com/kb/VI207
Good luck!