I've had many Casio ABC watches, so I'm biased. For this review, I had Casio PAW-1300t and Suunto Core (green version - latest batch) which are used interchangeable or together for a period of about 2 months.
1. The looks - the watch is big, but very conservatively designed. No weird plastic extrusions, just a simple watch with a big screen. It looks nice and professional. Casio looks a little more gimmicky, but Casio is still a solid looking watch (not like the older ABC casio watches that looked like spaceships)
2. The screen - it's nice to have a big matrix screen. Although, it may seem like a square pixel matrix, some of the pixels are actually taller than others - It's confusing for the graphs (if one tall-rectangular pixel is added is it greater difference than a reqular square pixel). Considering that there are 3 patches of matrix screens, I wish Suunto would try to do more with them (more combined data screens). There are 2 rows of tall rectangular pixels. Casio has a very small matrix screen for the graphs, but the rest of the screen is regular 7 segment digital display. It doesn't look as spiffy, but it does the job.
3. UI (user interface) - the menu system is organized like a tree. So you go inside menus to get to more menus, and then you have to back out to get back. It's a blessing and a nightmare - items are somewhat organized, but it's a hassle to get to some simple setting that's burried 3 menus deep.
3a. One big gripe with user interface: is that the barometer and altimeter are the same mode Suunto (which actually makes sense, given that both values are derived from the same pressure sensor). However, there's NO quick way to switch from one to the other. If you watch is in barometer mode, all you're going to get is barometer, if you want to change it to altimeter (or vice versa), you have to go into deep into the settings menus, find it, and change it. There's an automatic mode, but it's a gimmick, it tries to guess which mode it should be in (I prefer a more direct approach - I want to see what I want to see, not want it "thinks" I should see)
4. Clock Precision - the clock precision is outright terrible! It's a good 3-5 seconds a day. I've had cheap quartz watches that lose 5 seconds in a month. I'm no precision freak, but having my watch be off by a minute at the end of a month is unacceptable (if you're used to mechanical watches, this is not a problem :)
5. Instrument precision - pretty good. Many claim Suunto is better than Casio as far as accuracy/precision goes; I've had them side by side for a 2 months, and I don't see a difference. Barometer and altimeter are usually within the smallest measureable unit of each other (read: insignificant difference), and very close readings to the local weather station. Compasses show in the same direction. The compass on Suunto has failed me once; on a hike, it started twitching +/- 60deg (120-or-so degrees of total variance), I have not had that happen with the casio. Maybe I stepped into a magnetic anamoly - either way, I didn't like it. In general the Suunto's compass seemed twitchier than the Casio's (more needle jumping). Temperature is a useless sensor, my guess is that they only bother putting that sensor in, because the sensor is so cheap. However it works well in the water, but in air, it simply shows the temperature of the watch (which is greatly effected by your body temperature). You can take it off, and leave it for 10 minutes (if you've read this far, I'm sure you know the drill).
6. Features - Suunto doesn't dissapoint. The added benefit of Depth meter is very nice, even though it measure only to 10m (snorkeling only). The range of the pressure sensor is amazing (depth meter/altimeter/barometer are all run from the same pressure sensor). The data logging is very good; a little complicated, but very complete. The logs have the time and the measurement recorded at the desired frequency (casio, for example, only records the totals: ascend, descend, max, min; whereas suunto is a true logging device, and records actual altimeter data, every period). The sunrise/sunset display is very usefull (Casio doesn't have that). The storm alarm feature is a gimmick, it goes off in a seemingly random fashion - I don't know what algorithm they use to figure a coming storm, but it's obviously WRONG - as I had my watch go off on sunny days with no rains approaching.
At the end I chose Casio PAW1300T. As an everyday watch, it's just better built and better equipped, and it is smaller (much thinner). It's much more accurate (even without atomic connectivity), and on top of that, it gets automatically updated (to the second) every night. It's solar powered, so it doesn't need to be opened every 6 months (great concern for water-proofness in Suuntos). The modes are less cluttered(you still need a manual to learn the watch), and it looks more durable. Barometer and Altimeter have their own dedicated buttons (unlike the combined alti-baro mode on Suunto). I can't say Suunto is a bad watch, but it just didn't work for me. Casio is just as good at showing barometric trends (to predict weather). Suunto data logging is better than Casio's, but that's not saying much. If I need to log a hike to review it later, a GPS does a much better job (absolute 3d position on earth), so a simple altitude log produced by Suunto is too simplistic anyway. And as far as pacing yourself during a hike, the casio's log works just as well, without the added complications. It all came down to usability - what seems like a couple of button presses on Casio turns into a menu-down-down-enter-on-back-back-back kind of search on Suunto.