In terms of security, IMO, you cannot beat a properly secured PC - if for no other reason than notebooks and Ipads and smart phones often grow legs and disappear.
However, in ALL cases, the USER is responsible for security, not the machine. So if you buy from a reputable dealer, or build the machine yourself, you must ensure your system is fully updated, patched, scanned with a real-time anti-malware solution and blocked with a properly setup firewall.
Technically, for now, the MacBook may be a bit more secure, but only because there are not many MacBook users so there is little incentive for badguys to focus on them. But, that is changing.
If you buy a notebook, you do not need to reformat and reinstall. But again, you do need to setup your security after you get it.
I do recommend Windows 7 64-bit as it is more secure than the 32-bit version and Windows 7 itself is more secure than previous versions of Windows. I use and recommend Link Removed due to 404 Error and Windows 7 integrated firewall is excellent. And of course, you must keep Windows updated and you need to avoid risky behavior such as visiting illegal porn or gambling sites, or participating in illegal filesharing of tunes and videos via torrents or P2P sites.