There isn't a huge difference between the two. If your goal isn't insane FPS and you are keeping the resolution at 2560x1440 or lower. There isn't really any harm in dropping to the GTX 1070. There is about a 15% to 20% difference between the two. Even at 4K for the most part the GTX 1070 is fine. There are just a handful of AAA titles it can't handle at high settings at such a high resolution.
You could do a 1070 Ti with hardly any loss.
If I was doing a Ryzen build at that price. It would look something more like this.
PCPartPicker part list /
Price breakdown by merchant
CPU: AMD - Ryzen 5 1600 3.2GHz 6-Core Processor (£151.12 @ More Computers)
CPU Cooler: Scythe - Mugen 5 Rev. B 51.2 CFM CPU Cooler (£56.54 @ Amazon UK)
Motherboard: Asus - STRIX B350-F GAMING ATX AM4 Motherboard (£98.48 @ BT Shop)
Memory: Kingston - Predator 16GB (2 x 8GB) DDR4-3200 Memory (£182.76 @ More Computers)
Storage: Samsung - 850 EVO-Series 500GB 2.5" Solid State Drive (£112.74 @ Aria PC)
Storage: Hitachi - Ultrastar 7K3000 2TB 3.5" 7200RPM Internal Hard Drive (£39.60 @ Aria PC)
Video Card: Palit - GeForce GTX 1070 Ti 8GB Dual Video Card (£488.99 @ Overclockers.co.uk)
Case: NZXT - H440 (Matte Black/Red) ATX Mid Tower Case (£102.00 @ Box Limited)
Power Supply: Corsair - RMx 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£81.99 @ Ebuyer)
Total: £1314.22
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-03-25 19:43 BST+0100
or this
PCPartPicker part list /
Price breakdown by merchant
CPU: AMD - Ryzen 7 1700 3GHz 8-Core Processor (£231.59 @ Aria PC)
CPU Cooler: Scythe - Mugen 5 Rev. B 51.2 CFM CPU Cooler (£56.54 @ Amazon UK)
Motherboard: Asus - STRIX B350-F GAMING ATX AM4 Motherboard (£98.48 @ BT Shop)
Memory: Kingston - Predator 16GB (2 x 8GB) DDR4-3200 Memory (£182.76 @ More Computers)
Storage: Samsung - 850 EVO-Series 500GB 2.5" Solid State Drive (£112.74 @ Aria PC)
Storage: Hitachi - Ultrastar 7K3000 2TB 3.5" 7200RPM Internal Hard Drive (£39.60 @ Aria PC)
Video Card: Palit - GeForce GTX 1070 8GB Dual Video Card (£439.99 @ Overclockers.co.uk)
Case: NZXT - H440 (Matte Black/Red) ATX Mid Tower Case (£102.00 @ Box Limited)
Power Supply: Corsair - RMx 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£81.99 @ Ebuyer)
Total: £1345.69
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-03-25 19:48 BST+0100
Most likely the latter. Why? Expected lifespans. I expect a CPU and motherboard to give me five or more years of gaming life and ten or more years of total useful life. Those extra two cores and four threads can make quite a difference. Evidenced by the current differences between second generation Core i5 and Core i7. An i5-2500 isn’t very good now while an i7-2600 is still performs respectably well. When it is no longer useful for gaming. You can re-purpose it as a server, media center, work desktop, give it to a family member or some combination. An old high-quality motherboard still commands high resale values in the second-hand market. The price P45 chipset motherboards still command is insane.
Neither the GTX 1070 nor 1080 should be expected to have as long a useful life. When the GTX 1070 feels too old and slow, so will the GTX 1080. It isn’t just a matter of RAW speed. Something like the next DirectX version will likely kill their performance. If they can even utilize it. Other changes in hardware acceleration heavily effect older generations too. They won't have much use for secondary purpose such as servers or business desktop. Since they will waste too much energy without any performance benefit in daily tasks. When they could be sold and buy whatever the current low power option is out there for those machines. Like a GT 3030 or whatever naming convention nVidia uses.
As for the larger SSD. You can store a decent number of games on it. Once you feel the difference in launch times between an SSD and HDD. You’ll never want to use a HDD again for gaming. If you are using Steam. You can archive games to your HDD you’re done playing. In case for some reason you ever wanted to play them again without waiting for them to download. I don’t bother with the archiving myself. When I’m done with a game. It’s highly unlikely I’ll ever play it again. I have way too many unplayed games in my library. It’ll take me years to get through them all. Blasted Steam sales.