The primary difference between horizontal and vertical aquaria concerns the fish species that will suit either tank. Vertical tanks are no good for active swimmers (barbs, danios, some tetras, etc) because they need the length. Sedate fish tend to do fine in vertical tanks, fish like non-actively swimming tetras, some pencilfish, most rasbora, gourami. Some catfish are fine with either. Now, I'm not saying any of these in a 60 liter (15 gallon), just mentioning the restrictions on suitable species, in very general terms.
The vertical tank obviously has less surface area, both substrate and water surface, and this can be very important depending upon the species. This is why horizontal tanks are better suited to active fish--there is greater gas exchange bringing in Oxygen, in addition to the length for swimming.
A vertical tank with branches and floating plants (lower plants other than moss and ferns may have a light issue, and the sort of fish you want in these tanks do not appreciate bright light) can make a spectacular display for groups of small shoaling fish.
I don't understand how a rectangular 15 gallon, which would be 24 iches/60 cm in length, is much different in terms of space, but... .
I am not familiar personally with either, so going solely on the basis of appearance, I would be inclined to choose the Tetra. Reasoning is that the Fluval has a convex front glass, something I would find disturbing and difficult to view, having seen similar in stores, and it distorts. The Tetra is round, also not something I am fond of, but I would expect it to be better for viewing without distortion. Distortion in aquaria is a real issue that should be avoided.
Now, as for suitable fish...first, what are your water parameters for your source (tap or well) water? This is critical, as the small-sized species suited to this small a tank will most likely be wild caught. There are almost endless possibilities for soft or moderately soft water, and fewer if moderately hard or harder water.
Byron, you really are a wealth of information thank you! You are helping me massively!!
Because I would have to get a proper stand for it and in my sitting room I haven't got the space horizontally that I would if it was more vertical
I thought the tetra was cube but with a curved front?! I bought a 15 litre biorb and made the mistake of circle I wouldn't want to do that again (although I will make a little betts comfy so it won't be wasted) would it be like a biorb?
Ok so my water is medium/soft I've just googled it on the waters. Could you possibly give me a couple of examples of types of smaller fish that would be suitable in a tetra tank who were not as energetic as danios etc?
Sorry to be putting all of this on you, just you are seriously helpful!!
The dimensions of the Tetra are 33cm x 51cm x 51cmH which isn't much less space than a basic, but I leave that to you. When I said "round" I meant curved. But now that I think of this, it might be distorting as well. Curved glass with water always distorts what is on the other side...I'd want to look into this, see one "live" before deciding if this were me.
Do you have a number for the "medium/soft"? Assuming this is really on the soft side, the "nano" type fish are what you want, as this is only 60 L (15 g). The dwarf rasbora like Boraras brigittae (there are a few species in this genus, a couple are more often seen than the others), Ember Tetra (Hyphessobrycon amandae), pygmy cory (Corydoras pygmaeus) [or one of the other two "dwarf" species, Corydoras hastatus and Corydoras habrosus]. There is the Scarlet Badis (Dario dario) but this sometimes needs live food and doesn't always acclimate to prepared foods, maybe frozen only.