Are your 1080 cards amazingly cheap, or is your 1080 ti amazingly overpriced?
1080 ti normally runs more like 50% higher price than a 1080 of the "same model", not double.
The REAL debate would be "3 x 1070 ti vs 3 x 1080 vs 2 x 1080 ti" based on normal pricing I see.
I'm comparing Gigabyte GTX 1080 AORUS x2 (price at my local store is 710$) to Asus 1080 ti Poseidon (price at my local store is 1300). Price matter is only 120$.
Be careful taking replies here for granted, it seem like you got a few people giving misinformation there.
1080 is actually great, just not for eth mining. Both 1080/ti are a good choice for Zcash, but 1070/ti are better for ETH. For current mining Zcash scene, 1080 is more power efficient than all others. You can get 580 sols for 160w~ Its about twice the profitability of the other cards on ETH. Meanwhile the TI version can do over 700 sols with ease but at lesser hash per watts.
But if Equihash was to crash and you would want to go mine ETH, you would have difficulty with the 1080, while the 1080 ti would fare okay.
You will want to take everything you read here with lots of salt, fetch the price, watts and hash of the main algos and compare yourself on
https://whattomine.com