This is a good question. The problem is that I can't answer it 100% right since I don't have a G75 laptop with a 660M in it.
Someone told me that he gets 71°C under Furmark with G75 660M. I have a little bit more, I hope he was runing the 1080 as well (I never got the chace to ask). Room temp for me 33°C. All in all, it's not going that hot. OC to 755/1650 I get ~70°C in games. I find this OK, the thermal paste is stock. Someone in here gets 83 with a nice OC under heavy demand of Furmark:
http://rog.asus.com/forum/showthread.php?19580-Overclocking-670m&p=137155&viewfull=1#post137155. Just 12°C more it's not that much. In game it should be less.
Now Kepler sounds great, but it's not that easy.
My first idea was to get a mid Kepler card with Ivy and Optimus. So I got a 650M with 3610QM. Kepler should OC nicely, right? Wrong. At least not with Dell Inspiron. So instead of a cool laptop with great battery life, I got a heater (under heavy load) with average battery life and no OC options. This is just to show that Kepler itself is not that important.
Let's go back to Asus. I got a G55 660M and a G75 670M. They have about the same spec, 180W power brick.
The 660M can OC nicely to 670M benchmark performance in benchmarks. The overall system temp on the other hand is quite high. It can go higher, but you need to trick (by default it's locked at +135Mhz) it and the temp in G55 at least, under heavy load, might be just too much to consider this usable.
The 670M can OC to 675M benchmark performance with very little increase in temp (GPU and system itself). But do consider that G55 has a different cooling design so they can't really compare. You can go above 675M in game usage and ambient temperature around 20°C maybe. I think 675M is a good mark and at 70°C I find it OK. I only use OC when needed anyway. I play Inversion with low GPU settings to get 60°C since it can handle the game.
Now the thing is that benchmarks don't tell the full story about performance. I have Max Payne 3. So I have tried to play it with max settings on 660M. Stock, it won't. OC it struggles. On 670M stock, the game is moving a little bit better. OC the game is smooth most of the time.
Even if in bechmarks I have 660M OC = 670M, in real life software is more important. The cards are not exactly the same so you can't really say 660M OC is a 670M.
Difference in performance can be as high as 25% in benchmarks for 670M.
G55 has 2.5h battery life.
G75 has 2h.
Under load both have 1h.
G75 will use around 20W more power under heavy load, maybe a bit more. Idle/normal is about the same.
Both laptops are warm.
660M can be upgraded in theory. 670M not so much.
So, the better option? Well, people just started to hate Fermi for no good reason (I mean 660M stock at 71°C versus 670M OC at 83°C is not that much of a difference, is it? - I want to exclude myself for a moment here).
Keep this in mind if you want to sell this laptop anytime soon.
I think Fermi cards are very solid and will perform OK for a long time. From what I could see, there is some hype about 680M, but that is from another world of price tag, for me at least. 'Till I'll have the money for it, the 670M OC to 675M performance is what I can get right now. And since people don't want Fermi anymore, I got the G75 with 670M cheaper than a G55 with 660M in it. I think it was a good deal (Samsung has some 675M laptop at amost double the money). I remember not so long ago a 460M being the best I could get for the same amount.
If you don't want Fermi, but you want a laptop for a few years, I wonder how long will it take nVidia to release a Kepler card with 670M/675M performance. It might be end of the year for stocks to empty and for Holidays sales to help this.
Tech changes fast. What's best today, is old news very soon. Just buy something and enjoy it as much as possible.
