Having used both a little, I see no serious differences for what I gather you are planning. However, if you intend to get into serious data analysis, or would eventually like to move past the warmed-over SQL interface to the filesystem, I would go with postgreSQL. Despite the excellent documentation from MySQL that points out alternative techniques, I think supporting basic SQL concepts like sub-selects is important.

If you are going to be getting 100,000 hits a day, you might want to seriously investigate the speed differences with realistic tests of both systems on the same hardware. Other than that, the only reasons I can think of to go with MySQL are these: more widely used/supported, you have an application that you want to reuse that is built on it (and even with DBI, I can't imagine switching is as easy as changing a couple lines of Perl to point to a new db server in any but the most trivial applications), or you don't control the selection process (i.e. your ISP provides MySQL, but not postgreSQL). Otherwise I can't imagine why one wouldn't go with postgreSQL.