First off, as far as the PSU is concerned, you can choose any of the two GPUs (GTX 1070 or GTX 1060) with your current Seasonic S12II-520 PSU. So, for the PSU, you're good to go.
Now, the i5-3450 + GTX 1060 + 1080p/60Hz monitor would be a "more balanced" pairing as we have reiterated above. It will NOT give you a "more future-proof" setup because your CPU is already outdated (though still powerful) in itself. Having said that, if you want to game now (today; not 2 or 3 years later), the GTX 1060 would be the best in price/performance for you since you only got a 1080p/60Hz monitor anyway. That GPU can give you 50-ish fps on Ultra Settings on most games (but for graphic-intensive AAA gaming, expect your fps to dip around 30-40 on high-ultra). This option will save you a lot of cash today -- but don't expect the GTX 1060 to serve you well in the next 2 to 3 years with your current CPU, MB, RAM, and Monitor.
This is where your dilemma starts. If you want future-proofing (and eliminating bottlenecks), the GTX 1070 should be paired with a more powerful CPU (which means changing your MB and RAM), and a better 1080p monitor which can deliver more than a 100Hz in refresh rate -- so that you can achieve that 60++fps smooth framerates on Ultra Settings AAA gaming you want.
If you are low on budget, get the GTX 1060 is be content with adjusting in-game graphic settings to get acceptable framerates.
The performance difference between the GTX 1060 and the GTX 1070 is HUGE (so is the price). But getting the GTX 1070 now without upgrading your other components would not be beneficial for you.