There are always pros and cons with different GPUs.
100 watts at 300h/s sure is nice. But $150 for a 290/390 is much better than $400
Even if you pay alot for power, you are most likely better off with the 290 today since the speed today will yield much more profit than the power savings in the future. Because in the future, 6 months from now, who knows if there will be anything worth mining with either the 290 or 1070.
$150 for a 390? i guess secondhand not new, second hand 1070 should cost less than $400 already especially if reference, i guess even $300
if everything die i just sell back, with a 290 you are stuck there no resale value
if we assume a electricity cost of 0.2 cent it's far better to go with a 1070
6 x 1070 against 12 x 290 based on the investment($300 vs $150, secondhand), one would consume $86 per month the other $430(600w vs 3000w)
one would give you 0.36 btc the other 0.72(assuming same hash, 250 sol), so net for 1070 is 274-86 =$188, for 290 is 548-430= $118
so you see a 1070 is better at 0.2 cent, but not at 0.1 cent(you can easily check it)...not to mention that 3000w need bigger psu and better cooling, which add to the end cost
all depend on the electricity price