I'd say wichever your more familiar with.

Getting a PIX and then ****ing the config is no more secure than a Linux box.

Plus there are needs.

Do you need auth and crypt in PIX? or is just basic packet filtering + stateful inspection..

Anyways, if you wanna go for a rock-solid appliance get the Nokia series with CPNG pre-installed. Those come with a really hardened OS and allow the installation of other software <actually, only ISS's RealSecure> and are setup for routing purposes along side with a cute and handsome 'lil web-based configurator called Voyager

etsh911