There are several criteria which should be considered in the qPCR assay. In simple words, qPCR with cDNA is more sensitive and preferred over single step RNA.
qPCR with cDNA offers us the opportunity to control the assay and multiplexing. cDNA can be stored for long time, thus if there is no time restrains, this method must be used.
qPCR with RNA is fast method of analysis where the initial material is not the bottleneck. Nevertheless, the possibility of the degradation of RNA is the major factor in producing errors in results.
SInce, qPCR efficiency depends more on the expertise in lab and experience of user rather than the method itself, the beginner should always go for the qPCR from cDNA.
One step is much easier to perform but when you are obligated to make troubleshooting ..two step is much preferred. So, if the procedure you are goimg to do is consensus then go on for the one step ..if there is any doubts go for the two step.
Basically, in the molecular field, the greater number of steps, the greater probability of errors, especially with the poor experiences. So one-step is my choice.
One step is preferred if you are quantifying and any petting error (difference in volume ) can magnify and can give false results. One step also can be multiplexed.You save reagents and also time. Less errors.
On the other hand two step gives you liberty of storage of cDNA for long time as it is stable and can be used if stored properly. In the cases of follow up where you may require to run original sample along with your test sample, cDNA would be preferred choice.