Recreational tables suck. They are way too course, especially as you go deeper, and if you drop a few feet to see something cool, or recover something you dropped, you can screw your NDL to the point where you have to miss part of your dive.
+1 I'd definitely get a computer over any other piece of diving gear if you're not buying everything (Regs, BCD, etc). Not having to use tables and having instantaneous NDL time while underwater is a huge benefit. I say if you're not buying everything because usually if you're going to rent there is a bundle deal which includes exposure suit, regs, tanks, etc and the cost benefit to having a couple of these pieces isn't worth it (esp when you factor in servicing). That being said, it's still nice to have you're own stuff so you know how it performs and you know what to expect. However, when money is a key factor, I'd go computer all the way.
As for the type of computer to get, really most low cost computers will do what you need it to do for recreation diving. Features I'd keep in mind which drives the cost up a bit:
1)
Nitrox - Even if you aren't diving it now, it's a really good feature to have for the future. Most computers have it except the really cheap ones. Prob hard to resell without it.
2)
# of buttons - Cheaper ones have less buttons which makes them more cumbersome to use. One button computers are kinda a pain as you need to hold them down to access certain functions whereas a quick tap will do something else. More buttons = more money. I would never buy a 1 button computer.
3)
Air integration - Definitely a nice feature to have in the future (if you don't have a transmitter to start you can buy one later). I just started diving with air integration and it actually helps me with my air consumption as I can see breathing improvements in real time.
4)
User replaceable battery - A must have!
5)
PC Download capability - A nice to have although not an absolute necessity IMO.
The Hollis DG03 you mentioned looks like a good one. It hits the first 4 points I mentioned which will give you some room to grow (not sure on PC download capability).
If you are looking for something cheaper the Suunto Zoop like the other guys mentioned is a solid computer. I used to dive with a Gekko and have nothing but good things to say about it. Recently upgraded to an Atom 3.0 just for air integration and not because there was anything wrong with my Gekko.
