The WA2000 has better damage multipliers, configured in such a way that Stopping Power (an extra 1.4x damage multiplier) is not necessary nor helpful; it does 70 base damage unsilenced, with 1.5x multipliers for the head, neck and chest (meaning a one-shot kill in these locations, regardless of Stopping Power), and 1.0x for the stomach and limbs (meaning it will never get a one-shot kill here, even with Stopping Power). It's a good gun to use if you want to use a different tier 2 perk, like Cold Blooded. Of course, if you use it silenced, you need Stopping Power to get back to 70 base damage.
The M21 EBR does the same 70 base unsilenced damage, but with weaker multipliers; 1.5x for the head and neck, but only 1.1x for the chest, meaning that the chest will never be a one-shot kill unless you use Stopping Power (70 x 1.1 = 77 damage), and the limbs are 1.0x.
Of course, this is all negated if you're playing Hardcore, as all sniper rifles will get one hit kills regardless of perks or attachments, as the minimum base damage is 50 with a silencer, and in hardcore players only have 30 health.
Personally, I've been using the M21 EBR with Cold-Blooded in an attempt to force myself to aim for the neck and head rather than the body. So far it's the only gun I've gotten Mastery for.
Long story short...
For the WA2000:
* Base damage is 70, or 50 if you're using the Silencer
* Multipliers: 1.5x for the head, neck and chest, 1.0x for stomach and limbs
* Unsilenced, it can get one-shot kills on the head, neck and chest, and does not benefit from Stopping Power normally
* Stopping Power is required if you use the Silencer, as it brings the base damage from 50 back up to 70
For the M21 EBR:
* Base damage is 70, or 50 if you're using the Silencer
* Multipliers: 1.5x for the head and neck, 1.1x for the chest, 1.0x for stomach and limbs
* Unsilenced, it can get one-shot kills on the head and neck without Stopping Power, and the head, neck and chest with Stopping Power
* Stopping Power allows for one-shot kills to the chest, which wouldn't normally happen