It really depends on how good your aim is and how aggressive a player you are. The DSR has the larger OHK hitbox but it also has a slower rechamber rate which can become an issue when taking on multiple enemies at once. It's best to keep some distance between you and your enemy when using the DSR to allow for the slower rechamber time. The Ballista has a small OHK hitbox (requires good aim) but rechambers really fast which makes it ideal for taking on multiple enemies in quick succession and works well for closer engagements.
I use the Ballista Iron sight, I hate the DSR. If the DSR could use iron sights then maybe i'd use it.
There really isn't a best sniper rifle. 3arc did a nice job of balancing them so there isn't a clear dominant winner. It all depends on your comfort level with the weapon and what the class you build with it is designed to do. The balista with it's faster handling is the clear winner for quickscoping and close range snipe fights. However, the DSR can function in a similar role and its large 1 hit kill area makes it the better for longer range engagements and more traditional hard scoping. It really comes down to personal preference.
My personal preference on the sniper rifles is the SVU-AS. I know I am in the minority on this opinion, but I am not a traditional sniper or quick scoper. The SVU has the largest magazine, fastest fire rate, fastest reload time, and the least recoil and idle sway of any of the sniper rifles. However, it sucks butt in the leathal department and normally requires 2 shots to kill without a head shot. It also has a lower zoom level on the default scope which, makes it more ideal for mid range combat than the other rifles without the addition of other optics.
Most players who run a sniper rifle run it as their primary weapon with 2 or 3 attachments. I normally run the SVU as a secondary weapon on a pistol or shotgun class. Normally, I will run the weapon raw using overkill in my secondary slot or just stick it in my primary slot if I am using a secondary weapon as a serogate primary. The weapon is very competative at middle and the first half of long range against anything but another sniper rifle. It's lower recoil and idle sway mean I don't need the balisitic CPU or other optics to make the weapon functional and thus it minimizes the number of slots I need to have a long range weapon.
The other role I put it into is with FMJ as a back up weapon. I have it as a secondary weapon on a couple of classes where my primary is running an MMS sight. If I see somebody camping behind a wall, I can swap to the SVU and spam off a clip through the wall at them. The higher rate of fire means I don't have to worry about getting a 1 hit kill shot on a target I cannot see. With FMJ the SVU is always a 2 hit kill through walls and it shoots fast enough most players die before they can react to this tactic.
SVU, because I am a terrible... terrible sniper :-P