Hi Linda,
I have not used a Mares Nemo, so I cannot comment on its function. I'm using a Suunto Gekko ( done Nitrox on it ) for about 1 yr, and it had served me well. Forget about the mossie, had a friend who regretted getting it. Buttons too small, too stiff. The screen is too small.
Why I decided on getting the Suunto brand of DC was because, I had dived with a friend's Gekko before buying mine. Suunto's DC are quite easy to use, quite Idiot proof so to speak ( I'm referring to myself
). Setting it to Nitrox or Air mode , setting Max depth alarm, retrieving dive data etc . And alot of Suunto's DC have battery that are user replacable. That is, able to buy the battery off the shelf and replace by the diver. ( To be prudent, send the DC in for pressure testing to ensure the O ring are properly seated ) Unlike some brands whereby the diver needs to send in the DC to the service centre just to get the battery change, provided there IS an service centre for the brand in your country, else you will need to pay for 2 way freight .
Had this piece of advise from a fellow diver when i'm shopping for a DC. He said, "look around when you go diving, see what is the brand of DC that most divers are using." Well in my part of the world ( Singapore ) there are more Suunto users than any other brand. And he mentioned that, if as a new user you runs into problem using the DC or understand its function while on a dive trip and manual is not within reach, at least , you can be sure that, there will be some divers who are using the brand of DC you are using.. And able to get some help from them.
After deciding on the brand, I shortlisted Suunto Gekko and Vyper. For me, i will not be doing Deco dives, ( so I do not need Guage Mode ) . I need a Nitrox capable DC. And night dives constitute less than 20% of my dives . ( So I do not really need the backlight. ) Hence I chose the Gekko over the Vyper. The S$100 i saved from buying a Gekko, I used that to buy a 6watt LED dive light, ( my primary SL4 now becomes my backup light ).
There is this Suunto website whereby you can choose the different models and compare their functions side by side.
http://www.suunto.com/suunto/main/article_1column.jsp?search=search
Sorry for the long post.
Let's see the views of other users on their prefered DC