A Mac can run Windows too, but a Windows laptop cannot run mac Os (because mac os can legally be installed on a mac only)... So, if Mac OS is a must, you should go for a MacBook...
Now, first, we should know what is your cousin planning to use the computer for... Mac has some awesome software for multimedia editing (I think), and they are not available on Windows... (ask Aryayush, nepcker and goobimama for more details and clarifications)... Other than this, if you need regular word processing, spreadsheet, presentations, browsing, etc, Mac is well-equipped to give you a lovely experience... If you need to use Windows software on a regular basis, you will have to run Windows by bootcamp or Parallels on the Mac to achieve it (but the experience may not be so good if they are graphic intensive programs, I think)...
Customising Mac - are you talking about mac the laptop or mac the os? The laptop can be customised with RAM, HDD, etc - this is basically the most other laptops will do too, so there is no difference in it... With Mac the OS, you have a different story... In Windows, you have so many customisation settings and tools that let you tweak the look and feel of the OS. AFAIK, Mac OS is not that much customisable (you have themes etc but if you are a hard core Windows user, you will know the level of customisation I am talking about), but you probably wont feel the need for it...
Some of the statements I made regarding Mac may be my preconceived notions, therefore, if there is any error in my statements, I request aryayush, nepcker, goobimama, etc to point them out...
Now, the fact that your cousin seems willing to shell out extra for the Mac seems to imply to me that he is (at least vaguely) insterested in learning about Mac... If he doesnt mind the extra cost, he can try it out... After all, he still can dual boot with Bootcamp and run Windows if he is displeased with Mac OS...
Arun