In terms of heat and power consumption HD 7750 produce less hit because it's uses
28nm tech so it is more efficient and produce less hit and require less power .
if you look at tech spec of 6670 on below link you will find that 6670 have more core speed 850 MHZ which is 50 MHZ more then HD 7750 .
Universal Shaders: 800 of HD 6670 where as HD 7750 has 512
Texture Units: 40 of 6670 where as HD 7750 has 32
so if you want
more universal shaders+Texture Units+core speed + memory speed + less to spend =6670
or if you want
less hit + less power consumption + spend more money=7750 http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-3.html