FWIW, I work on a project that requires, among other things, memcached and postgres support.
The version of PHP 5.3 bundled in Snow Leopard doesn't have postgres support compiled in.
Installing memcached against MAMP is non-trivial -- its version of PECL doesn't work. There's a blog post which includes a link to a pre-built memcache extension binary, but it's only compatible with a single old version of MAMP.
The Mac Ports version of PHP, on the other hand, fairly trivially supports postgresql and memcached. It does not, however, have a pretty interface like MAMP -- you'll be spending some time on the command line installing and managing it.
Haven't tried XAMPP. It may be worth a shot.
In summary, if you have a simple set of dependencies -- you don't need any extensions -- and you're not comfortable on the command line, go with MAMP.
If your project has many dependencies (multiple PECL extensions, etc.) Mac Ports may be your only option.