Look at the damage, not the DPS. DPS takes the number of times you can fire per minute, the average damage per hit, then divides it by 60. So sometimes photons show a higher DPS than quantums, but the damage number will show you if it does more per hit than photons, which in most cases it will.
Most people prefer quantums due to the fact that you will not be repeatedly firing torpedos every time it refreshes. Instead you'll use them at the best opportunity. So what do you want, a 2 second faster reload or a huge hit? When their shields are down, all it takes is that one torpedo.
Another Question: Can you buy them in some shop? The prices in exchange are far too high...
Yes you can, as a Lt. Cdr from 2nd floor Shipyard. However, it will cost you 77 2nd Exploration badges from Hromi cluster(?) once you hit Lt. Cdr.
There'll also be a mission that rewards you a Quantum Torpedo Launcer.
DPS highend (photons in this case) for pve more spammable damage vs Mobs, but when in a more pvp orientated fight Quantums for the Big hit when their shields drop. Less chance to survive the "vulnerable state"
As a personal choice, Ill use Both further down the Tier ship lists. Solid Mix you know!