If you already have GPU, you should look into offsetting perhaps with an ASIC for Litecoin or maybe an A3. 

Those costs have gone down. 

However, I agree with above posts.  The benefit of GPU's and their associated PSU, memory, HDD, etc. is that they have resale value.

I have a 1080 I somehow got at newegg in January for $549.   Used on eBay today they are worth, $635. 

I am selling it because it will offset the cost of the other 5 GPU's I have while the price is lower.   It also doesn't hash as well out of the box as 1070's.

Personally,  I prefer AMD RX series.  They are much easier to use in a windows OS.  If you are using SMOS or something like that, you'll be fine with NVIDIA. 

If you are using Nicehash, then go with NVIDIA.    Using AMD RX you get an error if you use the most updated version of the RX series drivers.  They are incompatible with the nicehash legacy program. 



You could build a rig with 1-2 of the NVIDIA Titans.   Those would have even longer resale value than the 1070 or 1060's.

You're hash rate would be lower but in the event of an all out  crash back to April 2017 prices....those things would hold their value a lot more than the flood of 1070's or 1080's that would fill up ebay.

my 2 sats PT