P-valves are significantly better, in my opinion. A blueprint for an inexpensive home made version is available in CDG Newsletter 146 (pages 3-5), with additional notes in Newsletter 221 (pages 14-15). 

you may find its not so much the length of the actual dive that is important in this debate, but more the amount of moving about, climbing over obstacles, etc whilst suited up before or between dives, that is the key factor. P valves are pretty sound, but it is the catheter that is the bit that works loose with movement. For a project I have which involves a lot of dry-suited caving between sumps, I adopt a belt and braces approach of P valve plus nappies - I found a ' teenagers incontinence pant ' ( or rather my wife found them in a spanish supermarket ) that fits me fine and mops up any spillage that gets through the catheter. This is only really important if you are needing to leave your suit in  place in a cave for a while. Otherwise you can easily dry it out in the sun.