Yes there is. GTX 1660 is GDDR5, while ti is GDDR6 meaning the ti is faster in terms of graphics memory.
Also the increase in gaming performance is higher than the GTX 1660 by 10% to 25% depending on the game.
Pricing wise, GTX 1660 cost 219$ while the ti cost 60$ more. That being said, you can find them cheaper on the internet. While the ti performs better, the 1660 is a great gaming card as they both run most high-end games over 60 FPS.