Well, I've played both the FIFA 2004 and the PES 3 demos for PC..... and there's only 1 conclusion...... PES 3 wins hands down!!!
They are only demo versions, and there are a few things to fix before final release. For PES 3 the ball sound is too loud, too unrealistic. I experienced slow down as well in the demo. But just try turning antialiasing off for your graphics card. This cured the problem completely for me. PES 3 demo runs silky smooth at maxed out settings on these specs:
ATHLON XP2600+, ASUS A7N8X Deluxe v.2.0, 1.0 Mb PC 3200 RAM, GForce 4 Ti4600 128 Mb.
The game speed in PES 3 demo is too fast though. They look like they are on drugs zipping about everywhere! This will surely be toned down a tad in the final release. Also, I use a USB converter so I can use a Playstation 2 gamepad. The buttons are all mixed up in the demo, but you can easily change them.
The demo is quite fleshed out too.... you can play a full game, on any of the 5* settings, and choose from England, Germany, Spain or Italy. Much better than 45 mins. of Juve v Inter!
There is much more variety of goals in PES 3 too. So many different ways of scoring compared to FIFA.