I also own both a Casio 5600 and a Timex Ironman (30-Lap). From a functional standpoint, the Timex is far superior IMO. The Timex display is larger and easier to read, the menu system is very intuitive, and the features of the Timex are also much more abundant. The Timex has multiple time zones, alarms, and an occasion reminder. Timex buttons are large and easy to press. The FLIX activated illumination is cool.
That said, I actually prefer the 5600! The build quality feels MUCH more solid. The Timex feels really cheap and flimsy (eg crystal and caseback are thin). The Casio feels like a watch that will last much longer. With my Mil-Shock modded 5600, I think it is more comfortable to wear. The big plus with the 5600 is that it displays the time in CDT and chrono modes. The 5600 also looks better to me (classic timeless style) and has just the basic features that I really need. I don't really need the extra features of the Timex.
I wound up returning the Timex (still have another one as a beater at work). I tried on a Timex Shock model but it was too big for me. The metal bezel looks like it could scratch really easy.