The 390 makes sense over any 3-4gb in the same price range. You can buy a new one now for around £240 and if/when you need extra power in a yewars time you can easily buy another 390/X used for probably £150 and have a system that beats a single TitanX in 4K for a total of less than £400. Arguably the best financial decision right now for those on a budget.
The 970 has 3.5GB useable so will not really be any use for SLI and would need to be upgraded when the memory is not enough.
My current 7970 crossfire was basically bought using the same methodology and currently is faster than a 980/390X for performance and comes close to a 980Ti when overclocked. If anything is limiting it, it's the 3GB VRAM in games such as GTA5 and dying Light where stutter can occur when settings are maxed.
The 970 has 3.5GB useable so will not really be any use for SLI and would need to be upgraded when the memory is not enough.
My current 7970 crossfire was basically bought using the same methodology and currently is faster than a 980/390X for performance and comes close to a 980Ti when overclocked. If anything is limiting it, it's the 3GB VRAM in games such as GTA5 and dying Light where stutter can occur when settings are maxed.