If you are looking for higher quality enlarged images, say 8x10 and up, then the combination of 10 mp, 5x optical zoom and image stabilization give the nod to the Pentax. Even though the megapixel count is just about dead even the better optical zoom of the Pentax will mean higher quality enlargments than the Olympus since the Pentax can zoom in tighter on the subject, near or far. Also right now SD-HC memory is cheap, very large capacity and speed wise smokes just about all other forms of removable memory including the xd used by Olympus.
Other than that I'd say it's a horse race between the two makes. Both are excellent cameras with pluses and minuses. I would say you will be pleased no matter which make you decide to buy. I must also say that I have always been biased towards Pentax.