I agree with the above, it is easier to keep both your cylinders the same. It certainly is with the way I have my kit set up. Having said that, a steel 15 and a steel 12 have almost the same bouyancy characteristics due to water displacement, so no need to mess around with lead. Also, a 15 L cylinder is a bit of a lump to lug around out of the water. I find the 12s much easier.
Re: brand. I really like the PD cylinders because they are heavier and longer than Fabers which suits me.
I don't think there is a right answer to your question. Just depends how inconvieniant you find the 15 L and your gas requirements for your typical second dive of the day.
I have a 15 L PD cylinder in very good condition which has just been hydro tested. Would sell if you're interested. I'm based in Tutukaka/Whangarei.