Metal halides typically punch deeper into the water column, but do create a lot of heat. Halides will also get you a shimmer effect which I (and I think most people) like. Looking at the picture, it looks as though that fixture has 4 T5s in it, so you're going to get more light out of that I think than a 6 bulb T5 system all other things being equal.
Odyssea offers inexpensive Chinese-made equipment. Quality seems to be hit and miss all over the board. Some questions I would be intersted in knowing before I would buy used are:
How old are the bulbs?
Are they the stock (original) bulbs?
If the bulbs are stock bulbs older than 6 months, you'll probably want to replace them in short order, which will run you about $180 or thereabouts. If they are aftermarket bulbs, see how old they are and what the brand name is. (Geisseman and ATI are good T5 manufacturers, Phoenix seems to be the gold standard in halides. There are some other good ones around as well.) If they are older than 9 months, you need to plan on replacing them soon.
If you're going to have to replace bulbs soon, for just a little more cash, I bet you can buy a pair of LED
fixtures which will give you shimmer, and you shouldn't have to replace for 10 years or more. there are several individuals on the forums who have had good things to say about the Taotronics units.