ETH hash algo is much better on nvidia but if used dual mining then radeon is the way to go.
Error.
ETH runs faster on AMD than anything NVivdia, though NVidia can argue hash/watt especially on the GTX 1070 against even the RX 480/470 (pretty much a tossup there) and the 1070 is fairly close on hashrate vs the top AMD ETH cards - at TWICE THE PRICE per card.
For pure hash, the R9 290 / R9 390 are still the kings, though the 290x / 390x are pretty much the same if you're willing to put up with their even higher power consumption - the extra cores do NOT help on ETH.
384 bit wide memory access seems to make up a LOT for the slower memory speed vs. newer stuff.
GTX 1070 DOES seem to be the sweet spot in the current NVidia lineup for almost any sort of mining where NVidia is at all competative.
I haven't crunched numbers on XMR enough to have a strong opinion there, though I suspect some CPUs might be the sweet spot on hash/watt there.
No clue about XPM, even as far as what that coin is actually named.
3 card rigs make sense if you're putting them in cases, from a "ease of keeping all 3 cards COOL" standpoint.
Seasonic actually isn't all that expensive when you start comparing against other QUALITY gold-rated or better brands.
They does tend to have a small price premium against EVGA most of the time at the same power-rating and efficiency level, but if you wait for them to go on sale.....
Enermax for me has been a mixed bag - have one of their Platinum units in my "big rig" that's been working well, but have had too many dead ones on other models.
BTW - not sure what you mean by "go for the green" since pretty much EVERYTHING any more (except power supplies) uses the same small selection of chipsets, the same or VERY similar UEFI bioses, etc.