Short Answer: Boundless Ages and Dark Radiance are your best choices, if you remember to use the on-use one appropriately.

In order of DPS I believe it goes:

Dread Guard Relic of Boundless Ages > DG Relic of Dark Radiance > DG Relic of Kinetic Tempest > EWH Relic of Boundless Ages > other DG and EWH relics > Matrix Cube

Long Answer:

Dread Guard Relic of Boundless Ages is your single best bet, as long as you remember to use it relatively on-cooldown and during proper burn phases (or whenever you get the most effect out of it, such as full duration with Mara buff).

The Dread Guard Relic of Dark Radiance (internal damage) and Dread Guard Relic of Kinetic Tempest (kinetic damage) work off of tech crit, and are generally intended to be used with Imperial Agent and Bounty Hunter classes. Internal damage bypasses armor, and will do more damage for classes which don't have a lot of passive armor mitigation (such as Snipers). Kinetic damage is reduced by boss armor, but for classes that have a lot of passive armor penetration (such as Arsenal Mercs with their armor-piercing cell active) it will come out ahead.

The Dread Guard Relic of Elemental Transcendence (elemental damage) and Dread Guard Relic of Cerulean Nova (energy damage) work off of force crit chance, and are generally intended for Sith Warriors and Inquisitors. Similar to the above two, elemental damage bypasses armor and energy damage does not.

If you don't like the Dread Guard Relic of Boundless Ages (many people forget to click it during fights), I would recommend getting one War Hero or Elite War Hero Relic of Boundless Ages along with the Dread Guard Relic of Dark Radiance. They are a bit less DPS in ideal situations, but provide a constant and passive bonus to your Power and are very close to the same overall DPS. The biggest downside is that they can't be clicked for an extra boost with other buffs or burn phases.

Now is the time to buy these on live. Don't wait until 2.0 comes out or it will cost you more.

Daily Commendations will convert at 4:1 (currently) to Classic Commendations. The DG relics are 150 classic comms in 2.0 which is equivalent to 600 daily comms on live -- twice what the DG relics cost now. Of course, you will get classic comms from tionese, columi, and black hole comms too -- I guess it all depends on how much gear you would want to buy with classic comms once the change goes live.

Now is the time to buy these on live. Don't wait until 2.0 comes out or it will cost you more.

 

Daily Commendations will convert at 4:1 (currently) to Classic Commendations. The DG relics are 150 classic comms in 2.0 which is equivalent to 600 daily comms on live -- twice what the DG relics cost now. Of course, you will get classic comms from tionese, columi, and black hole comms too -- I guess it all depends on how much gear you would want to buy with classic comms once the change goes live.

The version you get in 2.0 will also be worse, at least judging by comparison on my sniper (had a boundless ages relic on live, transferred over and compared it to the one for sale.) I don't have the numbers off the top of my head, but the 2.0 version doesn't have the constant power bonus (around 4x something power I think, not sure on what the x is), and the 2.0 version gives 330 power when activated instead of 350.

(This assumes they allow the current live version relics to keep their stats in 2.0, they may change that later)

The version you get in 2.0 will also be worse, at least judging by comparison on my sniper (had a boundless ages relic on live, transferred over and compared it to the one for sale.) I don't have the numbers off the top of my head, but the 2.0 version doesn't have the constant power bonus (around 4x something power I think, not sure on what the x is), and the 2.0 version gives 330 power when activated instead of 350.

 

(This assumes they allow the current live version relics to keep their stats in 2.0, they may change that later)

I highly doubt they will allow for two different versions of stats for the same item. We can probably expect that they will fix the DG relics to match the live ones before the expansion is released -- this assumes that the relics on PTS have incorrect values. Alternatively, if they do indeed intend for the stats to change to the high endurance version that we see on PTS, then I would assume that all of our existing relics would be changed to match. All of that is to say, you probably won't have control over the stats on the relics by buying it early or late, you'll just save some comms if you buy it early.