Being relatively new to the hobby I can only tell you what guidelines I've learned along the way.
+ 8-10 gallons of water per goldfish because they produce lots of waste. So you can only have one in those tanks you were looking at.
+ 1 inch of slimbodied fish per gallon of water. I guess you can count a cardinal tetra as a slimbodied fish. Since they max out at two inches, that would be 5-6 cardinals in those tanks. And nothing else.
http://www.liveaquaria.com/product/prod_Display.cfm?siteid=21&pCatId=895
Bala Sharks get to be way too big - up to 10" in size. You will have to get a much larger tank for them.
http://www.liveaquaria.com/product/prod_Display.cfm?siteid=21&pCatId=1177
By the way this is a very frustrating and expensive hobby. For starters you may want to check out fins.actwin.com FAQs to kinda get an idea of things.
I can't tell you which tank is better. I know nothing about the seaclear. I have the eclipse system 12. It's been running for approx 1 year. Fishwise it's fine. Plantwise it's not great. Acrylic scratches easily. There's also the (JBJ?) nano tank that's 12 gallons. It's glass - I'm not saying it's better, but that it exists. I'm kinda curious about it myself. Perhaps I would have liked it better than the eclipse, or perhaps I would still be cursing myself. If you get any of these tanks and have tropical fish you will also want to buy a good heater (ebo jager and visitherm seem to be popular choices here). I'm kinda frustrated about how things are going now, so I apologize if I seem rather brusque.
I think there are threads on this tank in the board that you might be able to find. You can also look up ideas for ten gallon communities - there have been threads on that as well (I find it easier to search this site via google
BTW, since phrases don't seem to work here, at least not for me (I think that's a phpBB thing).)