Macs are generally more expensive than PCs, both for desktops and laptops. But they are very reliable (it's a lot harder to crash a Mac than a PC), they tend to be better designed, and if necessary, you can partition the drive using Bootcamp (it comes with the Mac), or use a virtual machine (either Parallels or VMWare Fusion) to install and run Windows and Windows programs. Macs generally don't get viruses (but if you run Windows on a Mac, you will need the same type of protection you would on a PC).
If your Sony Vaio works for you, I'm not sure you need to switch, even though I am a Mac user and avoid PCs unless I have no other choice. If you have solid reasons for switching, then do so. You say you've tried your friends' Macs and already know you like them—that may be enough reason.
Mac OS X Leopard is the Mac Operating System (OS= Operating System). It is the current version of the Mac OS, with a new version called Snow Leopard due out around September. On the PC you have Windows XP, Windows Vista, or the upcoming Windows 7. On the Mac side they just decided to name the new systems with types of wild cats. (We've had Jaguar, Panther, Tiger and Leopard so far).
You can get Office for Mac if you must, but you may want to look into Apple's own iWork, which includes Pages (a word processing program), Keynote (like PowerPoint only better), and Numbers (a spreadsheet program). It's less expensive than Office. Another option is the free Open Office, which does have a Mac version, and can open and save as MS Office files. It is more complete than iWork, and it's free (you can get it for PCs too). You can find it at www.openoffice.org.
The Mac comes with iLife, which includes iPhoto, iTunes, iMovie, iDVD, and Garageband. You also get a basic text editor called (logically) Text Edit. It has enough basic formatting abilities that I've even used it for writing papers.
There are other utilities and applications that come with the Mac. One of my favorite utilities is a wonderful screen grabber called Grab. You can take a screen shot of the entire screen, of the current window, of a selected area, and even do a timed screen (say you want your cursor showing in a certain position, like on a menu).
Your Mac also comes with a backup program called Time Machine. You can setup an external hard drive with it to do automatic backups.
Of course you also get Safari, which is Apple's web browser, and Mail, Apple's email program. (Personally I like Firefox for browsing and Thunderbird for email, they are both free to download and use.)
As for other programs, it all really depends on what you want/need to do. Note that there are lots of freebies and shareware available, you can go to the Apple website and the Downloads page and find all sorts of stuff. Also through the iTunes store you can find lots of free podcasts for all sorts of things, including training/tutorials for various programs.
Hope that helps.