755m Sli is about 15-20% faster. Depends rest of the laptop, is the 765m in a significantly lighter/thinner chasis? If its the same chasis I would personally go SLI, but that may just be because I'm a performance junkie... The 755m SLI will perform about as well as the 770m.
depends what the situation is - what are your needs and budget?
answering your questions more directly - 765M is much worse but won't get nearly as hit - 755M SLI is in a heavy laptop and is more prone to overheating but is more powerful
The difference is SLI has a 1TB 5400rpm and 765M has 750GB 7200RPM HDD. SLI is a Y510P and has a 8gb SSD for what it's worth.
My friend recently made a similar descision, in his case he's waiting for the Y50 (Successor to the Y510p, thinner and lighter but not as good at gaming) however between the two I would definitely pick the Y510p with the dual 755m. That SSD will really help with the boot speeds. Also the difference between the 5400rpm vs 7200rpm will be negligible day to day.