Dont get either a common or a sailfin - the common will usually stay smaller, around 15" but can and may reach 18", but sailfins will get to 24" in some instances, usually at least 18". Putting one in a smaller tank and hoping it will only reach 12" is a really bad idea, I'd ignore that advice.
Many hypostomus species will only get to 12", I'd recommend a nice rusty plec (I think the new L no is L310 but I'd check that on Planet Catfish), but they are slow growing, and apart from the rusty - fairly hard to find as many LFS will sell commons as hypostomus sp when they're not. If in doubt count the dorsal rays (the spines in their dorsal/top fin), as if I remember rightly 7 rays means it's a hypostomus, anything more and it's probably a common.
Another nice one is the L001, they usually max out at between 10" and 16", 12" being the average - but it all depends on tank size and companions (as L001s, despite being beautiful, are territorial and not very good with other plecs).