there's this myth that small subs can't play low, and that big subs can't sound good. honestly when I very first got into audio stuff I fell for that too, but it's simply not true.
the size of the sub isn't what determines the sound quality of the sub. it's the materials & engineering behind it. larger subs have bigger cones, and thus a bigger moving mass. when you are talking about crappy, cheap subwoofers, they are going to have crappy, cheap suspension systems. That's where this whole "big subs have muddy bass" myth comes from. It's just like the suspension on your car, the suspension on a subwoofer can be good or bad. And a bad suspension trying to keep up with a larger moving mass is going to sound crappy. But if you spend money on good materials, and time on engineering the sub properly, the suspension should have no problem keeping the cone under control.
Give me $30 for wood to build a pure SQ box and I'd put my current 15" sub up against any smaller sub you'll find in a retail store.
Small subwoofers can play just as low. The problem is, small subwoofers are smaller than bigger subwoofers. So they're quieter. So they don't have the same output at lower frequencies. It's not that they are somehow incapable of reproducing certain frequencies. This is another myth that's hung around because of cheap, crappy subwoofers. There are plenty of 10" subs out there, and even 8" and 6.5" subs, that are capable of 110+db at 30hz. Most quality 8-10" subs can easily put that out at 25hz as well.
The other huge factor in this is the kind of box you plan on using. With any speaker, you can manipulate the performance of the speaker depending on what type of box you use. You can shift the range of frequencies it can satisfactorily reproduce up or down the frequency scale, tune for output peaks, and to match the amount of power you plan on using.
Some 10" subs require more airspace than some 12" subs too. It all really just depends.
Long story short, you need to figure out what you want, and what your reasons are for considering changing subwoofers. That's the starting place, after that you can narrow down what you need out of the sub (what kind of music do you listen to, space constraints, etc) and a price range. If It's a space issue, try going with just a single 12. If you're afraid of a 10" sub not being loud enough, well that really just depends on what you're used to and what you consider "loud".