The 2980W 39Y4714 shows a little over 94% efficiency at 50% load verses the 1880W 39Y7349 at your stated 87% at 50% load.
So, let me make sure I understand this correctly;
If we had an S7 that we knew consumed 1210 watts [And used a 2980W 39Y7414 with 93% efficiency with approximately 40.6% load] we should see about 1,301 Watts at the wall on a watt meter.
AND
If we had an S7 that we knew consumed 1210 watts [And used a 2880W 39Y7349 with 86% efficiency with approximately 42.0% load] we should see about 1,407 Watts at the wall on a watt meter.
105 watts difference. It would be about the same with 2 x S7's on each PSU I believe. Let me see:
If we had 2 x S7's we knew consumed 2420 watts combined [And used a 2980W 39Y7414 with 93% efficiency with approximately 81% load] we should see about 2,602 watts at the wall on a watt meter.
AND
If we had 2 x S7's we knew consumed 2420 watts combined [And used a 2880W 39Y7349 with 86% efficiency with approximately 84% load] we should see about 2,813 watts at the wall on a watt meter.
So, my math is showing about 105 watts extra has to be burnt to power an S7 with the 2880W verses the 2980W. If we had 20 x S7's and burnt 105 watts more per S7 with the 2880W PSU, that would be 2100 more watts burnt. The price difference between each PSU is approximately $75 to $85. Let's use the $75 difference...
Converting that 2100 watts (2.1 kW) more power into dollars when using 10 x 2980's to power 20 x S7's:
730 hours in a month x 2.1 kW per hour = 1,533 kWh's in a month
1,533 kWh's x $0.10 per kWh = $153.30 saved in a month [All 10 x 2980W's paid for in 9.785 months; the price difference between 2980W & 2880W made up in 5 months]
1,533 kWh's x $0.09 per kWh = $137.97 saved in a month [All 10 x 2980W's paid for in 10.87 months; the price difference between 2980W & 2880W made up in 5.435 months]
1,533 kWh's x $0.08 per kWh = $122.64 saved in a month [All 10 x 2980W's paid for in 12.23 months; the price difference between 2980W & 2880W made up in 6.115 months]
1,533 kWh's x $0.07 per kWh = $107.31 saved in a month [All 10 x 2980W's paid for in 13.98 months; the price difference between 2980W & 2880W made up in 7 months]
1,533 kWh's x $0.06 per kWh = $ 91.98 saved in a month [All 10 x 2980W's paid for in 16.31 months; the price difference between 2980W & 2880W made up in 8.15 months]
1,533 kWh's x $0.05 per kWh = $ 76.65 saved in a month [All 10 x 2980W's paid for in 19.57 months; the price difference between 2980W & 2880W made up in 9.785 months]
1,533 kWh's x $0.04 per kWh = $ 61.32 saved in a month [All 10 x 2980W's paid for in 24.46 months; the price difference between 2980W & 2880W made up in 12.23 months]
1,533 kWh's x $0.03 per kWh = $ 45.99 saved in a month [All 10 x 2980W's paid for in 32.62 months; the price difference between 2980W & 2880W made up in 16.31 months]
So it's better to buy the 2980W 39Y4714 if you are into mining long term. It should be compatible with the Finksy 2880W board, right? If so I may buy 39Y4714 model from now on.