Options to consider
Suunto D9

Display - Take the Scuba Dive Computer in your hand. Look at the screen, can you read all the information. Does the information make sense to you. Are you able to identify all the important pieces of information you need to dive safely. If you find there is too much information and it is too cluttered, it will unlikely work for you.

Screen Technology - The most popular scuba dive computer models have a grey background with black LCD. However, alternative displays, such as colour screens are emerging rapidly in the market. These dive computers are more expensive, but they can give you more information as well.

Backlight - If you want to use your computer at night, poor visibility, have a backlight function is indispensable. You are able to keep the backlight on permanently or only on demand. If you leave it on all the time, it has serious deteriorates battery live. An important consideration what button or button combination you have to press to get the backlight to work.

Alarms - Depth Alarm, ascend alarms, dive time alarms are just a few alarms available. They can help to make your diving safer (if you pay attention to your alarm). You won't believe how many divers don't even recognise that it is their dive computer making the noise underwater. Understand what alarms you can set, this means you don't have to continuously stare at your screen.

Menu System - Important, you need to understand the scuba dive computer menu system and have the ability to access it on the surface and underwater. You might need to make changes to a setting. The menu system should be easily accessible and manoeuvrable in a logical fashion. If you think you will require to take a manual with you all the time, then it is likely to be too complicated for you. Small hint: so fewer buttons, so easier usually to navigate.

Thermometer - Logging the scuba dive water temperature will allow you to build up a history of "how you felt" and what your temperature tolerance is. This allows you to adjust your diving, when to wear your shorty, 5mm semi-dry or dry-suit. 

Ascent Rate Indicator - One of the most useful displays on the scuba dive computer. Understand where it is on the dive computer and what rate has been programmed - 10 m/min; 8m/min etc. This allows you to control your ascent easier and takes away the mental arithmetic you would have to do watching your depth gauge and watch. 

Safety Stop - A key feature of scuba dive computers. Scuba Dive Computers alert you if you have mandatory and non-mandatory stops. The computer will normally dive of an audible and visible alarm if you ignore the stops. For safety reason, the dive computers will lock out and stop you from diving for 24 or 48 hrs. if you ignored a serious mandatory stop.

Gases - The entry level computers will only calculate your dives using air (21% Oxygen, 79% Nitrogen). Most dive computers will do recreational Nitrox - allowing an oxygen mix up to 40%. For other mixes above 40%, you will usually require the more advanced scuba dive computers, as they will incorporate elements such as gas switching (diving with more than one type of gas), but usually not Tri-Mix (Oxygen, Helium, Nitrogen). Tri-Mix computers and rebreather computers are at the top end of the scuba dive computer scale (and can cost some serious money)

Wrist vs. Console - Not too much of a debate any longer - as wrist computers have become very small (watch size). Consoles meant you had one less piece of equipment to put on, as it was already integrated with your SPG. In many cases, the console computer incorporated a pressure gauge, allowing air consumption and remaining dive time to be calculated. Nowadays, wrist dive computers offer this as well, but you require a transmitter to be attached to your first stage (Note: Not all wrist mount dive computers offer a transmitter receiver capability - Air integration)

Comfort - Are you comfortable with it when you put it on your arm or in your console. A wrist computer will be on your arm during the dive and likely between dives as well - if it is too big and uncomfortable don't do it.

Download - Another consideration is download capability and PC software. If you are interested in keeping and electronic log-book, analysing your dives, you may want to look at computers that offer that facility. In most cases you have to buy the download cable and software extra.

Compass - A nice little features, but not the most useful feature. Great as a backup feature, but we find that the normal compass is easier to use underwater.