I don't own a computer. I don't even have my OW cert yet (1 month to my class). But, I have been doing a LOT of reading on computers, trying to decide what to get for myself. So, all this is from what I've read, not personal experience:
The Suuntos use a very conservative algorithm compared to all the other brands of computers. I have decided to avoid any Suunto for this reason and for having read too many reports of problems with reliability and with uncaring customer service. But mainly for the algorithm. If I decide I'm always going to get out when my NDL gets down to, say 10 or 20 minutes, I would rather have a computer that lets me stay down longer and still get out with the level of conservatism that I choose. From what I have read, the Oceanic and Sherwood computers seem to allow for the most bottom time. And most of the Oceanic have a choice of 2 algorithms, one of which is more conservative than the other. And with either, you can still add in a "conservative factor" to make it even more conservative, if you want.
Also, the Zoop is a bigger size, as you said. Looking at it in person, it is bigger than what I want, too.
I think the Veo is a bigger computer - i.e. bigger than what you said you want. I think it's around the same diameter as a Zoop, but I think it's thicker, too.
The Geo is a nice wristwatch size. I haven't read about issues with reliability on the Geo, so of the ones you listed, it is probably what I would get.
However, other options I am looking at are:
Aeris Manta - $200. No gauge mode. No download. But, it's a nice wristwatch size. It supports Nitrox. Aeris, Oceanic, and Hollis are all part of one bigger company and I think the Manta has the same "less conservative" algorithm as the Oceanic computers. It comes in a lot of different colors. And it's only $200....
Aeris Epic - $385, including a wireless air integration transmitter. This appears to be the same as the older Oceanic Atom 2.0. I read a bunch of stuff with these having problems, including a voluntary recall from Aeris. My LDS says they have an updated model that they think has resolved the issues that the original ones had. I'm thinking of trying one of these just because the LDS said if I end up having a problem with it they will handle getting it fixed for me or they will give me a full refund on it. These were originally somewhere close to $1000. And it includes a wireless AI transmitter....
The Suunto D4i is $500. If that's your budget, you might also check out the Hollis TX-1, which is also $500. The TX-1 is a full-featured, tech-diving-capable computer. It supports wireless AI, if you want to also buy a transmitter ($750 for a package deal). It supports multiple gases, comes with a wrist strap and a bungee mount, supports Tri-mix gases (down to 7% O2/93% He), uses Buhlman ZHL-16C with Gradient Factors, and has a built in digital compass. It appears to be the same platform as the Oceanic VT4.1, but with more capable software. So, it's probably a bit bigger and bulkier than you're looking for. But, it seems like the most capable and feature-rich computer you can get for $500. Oh, and I think it comes with the USB cable, which a lot of computers charge an extra $100 for.
---------- Post added September 19th, 2014 at 11:02 AM ----------
I believe the Hollis DG03 is almost the same as the Veo 3.0 except is only has the buhlman algorithm and allows an additional gas. But for whatever reason, it can be found a lot quicker.
https://www.divegearexpress.com/computers/dg03.shtml
I have one but I only use it i gauge mode.
The DG03 is $250. But, with data cable and bungee mount, it's $370. At that price, I think I would probably go ahead and spend $500 for the Hollis TX-1, which comes with the cable and bungee mount and is, otherwise, a more capable computer. Notice, I said "probably". Since I don't think I'm actually going to buy either one, anything I would "probably" do is suspect right up until I swipe plastic.