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.
Byron.