I had the same problem too with choosing a wristwatch computer (see post "suunto D4 worth buying?")
I eventualy decided a D6 and a D4 (for the wife). I will pick them up from the shop tomorrow. as such i dont have any experience to make a comment. however, i did try to research a lot and some points why i chose suunto are,
- a lot of people use suunto, so chances are your diving buddies will more likely to have the same algorithm.
- its a well established brand in diving computers.
-also a lot of shops supply suunto products. for me this is important. I just moved from japan where i lived for 5 years. a lot of outdoor shops have suunto products (non diving) so if need service you can take it to a shop and they will deal with it (send it to manufacturer etc). especially in Japan where customer service is one of the best in the world, i dont have to find a dive shop which can be difficult in big cities.
- I quite like the way d4 looks (more than D6, but i wanted two tank gas switch option so opted for the D6 for myself). I also checked it at my LDS and it seems pretty solid, no flimsy feeling to it..
-from what i read on the net with gear tests etc, it seems mares is even more conservative than suunto (not necessarily a bad thing.) however, most of my equipment regulator, fins, mask, etc are from Mares so i do like their products.
-a crucial point, me and my wife use mac computers and the only dive software available to mac are for now suunto. and thats because some guy decided to help all us poor mac divers and wrote a neat software called MacDive (thanks where ever you are!!) Its a really nice program to download your dives and keep a log. and its for free!! (unlike suunto pc connection cable which is around 100 bucks extra. ). There's a program called Jtrack for uwatec computers but the application is not that nice compared to Macdive.
my limited advice would be to go through the manuals on their internet site and compare the two products. if there is a function that one has that the other doesnt and its important to you, then the choice is easy. if they are very similar (which I think they are) then other factors that one has to consider are very subjective ( style, size, weight, availability, buddies computers etc.). Although its nice to get ideas from others which is very reassuring. thats what i did as well
good luck in deciding which one to buy!!