WAMP stands for windows, apache, mysql, php.
The best advantage of using wamp is that it is easy to setup configuration in wamp. But when i started using wamp, some times i faced problems while running php code , may be it was my coding problem but what else. Mostly people talk that wamp is not good for beginner but i say that wamp is very easy to use and it is best for both beginner and advanced users.
XAMPP stands for x-os, apache, mysql, php , perl.
x-os means it can be used for any operating system. xampp is easy to use than wamp. Its cpanel is very nice and good for beginner and its start, pause and stop button work very nicely by functionality. You can add other services to xampp also.
XAMPP is more powerful and resource taking than WAMP.
WAMP provides support for MySQL and PHP
XAMPP provides support for MYSQL, PHP and PERL
XAMPP also has SSL feature while WAMP doesnt. If your applications need to deal with native web apps only, Go for WAMP.
If you need advanced features as stated above, go for XAMPP.
As of priority, you cant run both together with default installation as XAMPP gets a higher priority and it takes up ports. So WAMP cant be run in parallel with XAMPP.