The 660 Ti is overall faster. Although i'd recommend the MSI Power edition 660 ti as it gives better performance at £20 less.
There is quite a difference really between the two. For example on BF3 all maxed out a 660 will get 40-45 fps average whereas a 660 Ti will get 60 fps average
isn't the ti manufactured with the same components as the 670 and 680 ?
It has a lower memory bus and a few other little things but otherwise pretty much! A 660 Ti Overclocked can reach 670 performance or at least come close. The 670 is supposed to have a 10% difference over the 660 Ti