-Fran- :
240W of difference in a 12V rail is 20 Amps difference for the rail consumption. But I haven't found a single review where the 390X uses more than 300W unless *heavily* OCed. In fact, the average use is 275W. Under those numbers, most 550W-600W PSUs don't have any issues putting up with a 390X. OCed 980s can go all the way up to 250W as well.
In regards to software, that is debatable and fully depends on what the OP plans on playing. I won't go to either camp yet based on assumptions there. Performance wise, both are solid options to me, where the 390X comes in cheaper and given history, AMD has a better long run.
And you never want to downgrade HDD for SSD space, the money doesn't add up in terms of GB/$. I have to disagree on that front.
Cheers!
Those figures are from TechPowerUp, and show max power draw. Though not applicable for most people, max figures should be taken into account when buying a PSU to show 'worst case scenario'. I would not recommend running a 390x from a 500-550watt PSU to anyone, but for its fine for a 980. There is absolutely no denying that the 390x is an absolutely power hog compared to anything in the Nvidia line up.
Debatable depending on use case, yes. But overall, AMD has a much harder time with software releases. Why? Becuase its market share is tiny compared to Nvidia (bare in mind that a lot of AMDs revenue comes from console chips). As such, many companies put more time/money/effort into testing for Nvidia products. For example, many VR titles are badly optimized on AMD hardware because they are mostly made by small developers.
Were seeing more and more developers hardcoding Nvidia Gameworks into their games, which usually results in bad performance for AMD cards. Just look at the most celebrated game of 2015; The Witcher 3. Terrible AMD performance for months until patches addressed a lot of the issues. The most recent one I can think of is Gears of War Ultimate - which launched with such bad AMD performance that it made headlines. Even Forbes were showing videos of a FuryX running at 12fps.
There are some instances of Nvidia suffering issues, but the list is no where near as long, no where near as dramatic and they took no where near as much time to correct.
So what do we have;
A GPU with slightly less performance, almost double the power consumption, a worse history of optimization in some of the biggest game launches over the last year, slower driver releases and less software support. Is that really worth £40? Not if you ask me.
As for SSD space, of course it doesnt equal HDD space for GB/£. That's not why people buy SSDs. People buy them for there comparatively colossal speed. If the OP wants to swap out 1TB of HDD space to double his SSD capacity, then he should go for it unless he needs over 2TB of storage.