The Cree XP-G3 and Luminus SST-20 are close contenders. Naturally the prize of the parts and thereby the final BOM is importand, but likewise the efficacy and real world foton flux.

The final current consumption is a big consideration, since the switches and the controller has a upper limet. With that said, it is unrealistic to drive the LED´s a full capacity since this will lower the lifetime of the LED´s.    

The temperature rise is difficult to compare, but the Luminus SST-20-DR seem to have a lower Thermal resistance.

My overall verdict, looking at the datasheet and prices, is in favor of Luminus for RED. 

Why? The Cree XP-G3 RED does have a higher efficacy but also a higher thermal resistance. And it is twice the prize:

(Source: Cree XP-G3 - Luminus SST-20-DR-B120-V660).  A side by side comparison would be interesting, unfortunately the footprints of the two does not mach. The Blue SST-20 does come in two versions, one of witch has the same footprint as the Cree. Seems strange, but maybe a way for Luminus to bind their costomers to this particular part.  

Note: The datasheets of the two candidates is hard to compare, since the two manufakturens might have used different methods of measuring umol/watt output. To compare the efficiency, one needs to measure umol/watt with exact same current and distance (conditions). There are some noticeably different specs.: 

Here Cree gives a maximum mmol/J, which is staggering 12.0 mmol/J. But is this at 1500mA? To compare, the Luminus state a 700mA value of  4.82 mmol/J. The maximum drive current is 2Amp. So maybe this is the mayor difference. To make the choice even more confusing, the Blue Luminus SST-20-B-A120 has a thermal resistance of a whopping 3.5 C/W.  Where the Cree royal blue has a thermal resistance of 2 C/W. Here the two candidates does not differ so much in price. And my preferred choice would be Cree. Since the two has the same footprint, one can mix and mach. 

Any real world experience with comparing the two is most welcome.

Cree XP-G3

PPF Efficiency (PPE) at 350mA: Royal Blue(2.83 mmol/J) Photo Red (4.08 mmol/J)

Thermal resistance: Blue(2 C/W) Red (2.5 C/W)

Luminus SST-20

PPF Efficiency (PPE) at 350mA: 3.28 mmol/J

Photosynthetic Photon Flux (PPF) at 700mA:  2.99 mmol/J

Thermal Resistance (Electrical) Deep Red (RTH 1.8 °C/W) Blue (3.5 °C/W)

Note 2: Sustained operation at maximum current will result in shortened lifetime