Lumens per watt, lower current is better. Lumens per cob, higher current is better. It just depends on how much you want to spend. Any extra you spend up front will be saved over time. Over the long run you're better off using a lower current, but we're talking about cost over time and not "how much light". Figure out how many par watts you want, work out the cost analysis on a couple options that meet your requirements and then go with the most expensive option you can afford and feel good about. There's nothing wrong with running these cobs at 2.1 amps... 50% efficiency is pretty good. But when you figure in the electrical costs over 5 years the higher current doesn't actually save money. Figure it over 7 years and the lower current is saving money even though it costs more up front.
In a 3x3 you could run 4 cobs at 2.1 amps, 5 at 1.75 amps, 6 at 1.4 amps, 8 at 1.05 amps, all providing around 300 watts. In these examples the lower current options will provide more light at the same wattage, but you could also get similar output with less watts using a lower current option. For instance 5 at 1.4 amps will provide almost as much light as 4 at 2.1 and use 50 watts less.