When we start thinking about investing in a scuba computer, whether it will be an upgrade or our first instrument, we will need to consider available investment funds, desirable parameters, future diving career, area of application of this device, and buying options.
Let's discuss these critical points in deeper details to have clear understanding.
Consider your future path as a scuba diver Let's presume that you already know your future diving career, how often you will dive and where, and what will be an area of an application of new scuba computer.
If you are a recreational diver only and dive just a few times a year in tropical waters - probably you won't need expensive dive computer with advanced functions, and on the contrary - if you dive very often and think about trying free diving or going technical.
If you plan that after sometime you will progress in scuba diving mastership or engage in free diving activities - you need to consider investing in more expensive computer that has multi-gas capabilities, wireless air integration, digital compass, free dive mode, etc.
When you have answers on above-mentioned points - you may make a list of desirable parameters and functions for your next model.
Which specifications are desirable in your dive computer? Here you need to imagine which features you want to have.
These days scuba computers have all imaginable and unimaginable functions and easy satisfy each and every scuba diver! And several of them were mentioned below just for your consideration:

1. possibility to use some dive computers as fully functional digital wrist watches onshore;
2. possibility to build computer consoles with submersible pressure gauge and compass;
3. few dive modes - Air, Nitrox, Gauge, Free diving;
4. some of them have Dual Algorithm for liberal and conservative diving;
5. possibility of programming for up to 8 different gas mixes to dive with trimix;
6. wireless air integration;
7. adjustable settings of Personal Conservative Factor, Altitude, Deep and Safe stops;
8. colour display;
9. PC interface;
10. dive simulator;
11. integrated digital compass;
12. possibility to upload from PC colour or grey - scale pictures and maps into the dive computer;
13. user replaceable batteries, etc.
    

I think the best answer about specifications you may give after setting clear your future diving path.
Available investment funds This part is one of the most important! Dive computers today are packed with many features and we may get multifunctional instrument with several dive modes, dual algorithm, wireless air integration, possibility of programming for several gas mixes, etc.
And each additional function may increase price considerably.
When we have come clear with our future diving career and all features and functions - it is the right time to look inside our pockets! It may happen that after considering every option and selecting the "special one", we suddenly notice that we are short of money for particular model.
We should not worry! Let's look realistically again...
Maybe we do just fine by having a dive computer without wireless air integration, multi-gas capabilities and digital compass right now, like Mares Puck or Aeris Manta and Cressi Leonardo, Sherwood Insight or Tusa IQ-750 Element II! These models are user-friendly and reliable dive-buddies with very competitive and reasonable prices in our market these days! But if your wallet is thick enough and you are planning to progress in your diving career fast, you may want to invest in more expensive scuba computer, like Aeris Atom 3.
0 and Mares Icon HD, Oceanic OC1 and Suunto D9tx, Tusa Zen Air IQ-950 and UWATEC Galileo SOL with capabilities of wireless air integration, multi-gas options, possibility of heart rate monitoring, uploading pictures, integrated digital compass and much more.
Buying dive computer from e-bay Here are few words about this option.
I will not do this personally.
I don't want to make you thinking that this option is worse than others, we all have our own opinions and preferences.
Just consider that instrument that was bought from e-bay might have some glitches and it would fail under water - this could bring you troubles! And on top of that - manufacturer warranty may expire by the time of your purchase.
Models without audio alarms and backlight for the beginners It is my personal opinion only.
If you are a beginner in scubadiving - it is preferably for you to invest in model with audio-alarms and backlight in order to be alerted during certain dive conditions.
Beginners do not have that experience yet, which mature divers have, beginners do not know what to expect and when! So, if you are at the beginning of your diving career - select dive computer that has audio and backlight! After all research we have done so far and considering all options, we have collected all information that we need before investing in the best dive computer, according to our needs, budget and future diving path.