Well once you chose between HP and LP (high and low pressure), you then have to decide what size tank you want.
You have the two to choose from, a 68cu (1.1litre tank), or a 48cu (0.8litre). Now i don't need to explain the physical difference, one is bigger than the other, and accordingly weighs a little bit more. The size can be down to personal preference. Children and people trying to minimise their weight / size of their gat will usually go for a 48cu just because it's smaller, so more comfortable to get a good stance with. Most however generally go for a 68cu, just because the difference isn't a huge amount, and it means you'll be refilling less between games.
If you're playing something like a big game, most will prefer to have a larger tank, purely because having to refill means another death for the team, and for players who are playing race2/3 format, just because it can save them having to spend time refilling their tank
As for the tanks themselves, you'll need to look at what the tank is made of. I'd advise going for a carbon wrap tank, just because they're so much lighter.
Guerilla Air seem to be making quite a name for themselves, very light and reliable tanks!
Just don't get a steel tank, those things weigh a ton!