When I was researching this issue, I was told that 10 gallons in larger size fermentor could affect the beer flavor; i.e., the geometry was important. I really not sure that's true for such small volumes.
But, I 'played safe' and got two of the medium size Blichmanns. Now that I've used them (3 years) I'd do it again. Like you, most of my batches are 10 gallons (although I'll occasionally squeeze 12 gallons into the fermentor - make sure to use anti-foam.) And, if I make 20 gallons, I just use the second fermentor; but, it's more common to have a second beer in the second fermentor...that's REAL common.
Good luck.