Let me make this easier.
Power:
You need to use 770 24/7 a day on 100% for 1 year long in order to get 20 euro's more costs ( 30 dollars ) on energy level.
Power almost costs nothing for videocards.
In general sli works like this the best.
x80 cards always feature a new generation architecture, doing this for lower versions like x70/x60 do not always work and they can be rebrands etc and miss features.
2x 480 in sli = 680
2x 580 in sli = 780
2x 680 in sli = 980
In order to get the best performance for cheapest and going the sli way becuase of it, you always have to go like this:
Example:
Year 1: buy 580 3gb v-ram
Year 2: add another 580 3gb v-ram ( 680 releases with 2gb)
Year 3: play your games and sit this one out ( 680, 4gb releases + 780 comes out with 3gb v-ram )
Year 4: buy a 980 with 8gb of v-ram ( doesn't exist i know but you get the idea ).
This is the most effective way to upgrade your gpu solution, the only reason to not go this line of thinking = i don't want to spend money or want solid performance for cheap prices and you are fine with low quality settings in games. Or you have issue's with extra heat / want features on new generation of cards / more v-ram to work with.
Another rule i always apply when going the sli route is the following:
1) always buy the most future proof card if you plan on doing this upgrade solution. Is there a double v-ram card for a little bit more invest in it.
For example if you want to buy a 290x amd card now, and want to crossfire it later on then get the 8gb version, no matter if it makes sense at this day of age.
For example you buy a 580, buy the 3gb version, you bought a 780 ti when it came out = buy a 6gb version.
V-ram is absolutely crucial to keep as high as possible.
770 sli vs 970
770 4gb
The 770 is a rebranded 680 with a little overclock on it and 4gb of v-ram.
If you look at the list above the 680 in sli should perform around 980 levels through my logic.
Lets compare some benchmarks:
770 fire strike extreme : 3800 points
680 fire strike extreme : 3700 points
Basically that seems valid a bit overclocked 680 so it should perform about ~980 levels let see about that.
SLI 680 2gb fire strike extreme: 6100 points
980 fire strike extreme: 6800 points
970 fire strike extreme: 5800 points
Clearly in between of the 980 and 970, but we are talking about 770's which are overclocked 680's basically, so lets see how that goes:
SLI 770 4gb fire strike extreme: 6700 points
980 fire strike extreme: 6800 points
970 fire strike extreme: 5800 points
Bonus:
290x fire strike extreme ( fastest AMD ): 6100 points
sli 670 fire strike extreme ( because dude above mentioned it): 6100 points
Titan (6gb) fire strike extreme: 5400 points
Basically 770's will give you better performance then a 980 in sli based games and probably will be cheaper for most people to upgrade towards as the 980 cost a fortune.
970 isn't well build for future purposes at all
The issue with this card really is, it's butchered v-ram, it's like the 660 it has a terrible architecture that makes sure the last 500 mb of v-ram is unusable as it drops your performance down to a crawl. This means the 970 really only is a 3,5gb videocard. This also mean that the card is absolutely horrible to sli in the future with specially with more and more v-ram that is required by games. If i had 2x 970's in sli and i could choose a generation later to upgrade towards a 6gb v-ram 25% weaker card i would take it in a heart beat. it just isn't well builded.
We already have games that need a minimum of 2+gb of v-ram to even function at 720p ( ac unity ) and 1080p ( watch dogs), 2+gb ( mordor) and that will only get worse when times moves forwards. 3,5gb of v-ram in my view at this day in age is just to low to invest into. It's great for current games but next year you will be starved for any v-ram you can get your hands on, and yes 500 mb extra can be a deal breaker ( talking about all the 580 1,5gb v-ram users that have to upgrade to a 2gb card for unity because of that extra 500 needed ).
Conclusion
I would go for a second 770 gtx 4gb if can sli it and got already a 770 4gb in your case ( 2gb version i wouldn't sli ), they cost about 150 second handed with warranty these days which beats everything at this day of age together still on performance wise.
980's cost like 550 euro's so yea a big no no here, and the 360 for a 970 just isn't worth it then.