It's not just the number of bits that affects signal to noise ratio, though... It's also the complexity of the calculation; more gate operations and longer storage in memory leads to more quantum decoherence. And AFAIK, each type of calculation must be implemented in hardware, because reversible quantum gates must be used. This means you would need a specialized chip for each algorithm. I haven't confirmed this understanding with someone that specializes in QC, but I don't see a way around it, other than to combine many algos onto a single chip, sharing as many gates as possible, but degrading S/N further.