Go with the i7. Battery life difference will be marginal (maybe 10%) under light load (and im asuming that whenever you will be using the battery you will be using the computer lightly--no gaming, etc because few people game on battery). i7 mobile idles at around 3-4 watts. Cutting it down to an i5 much change that to 2-3 watts. There will be a bigger difference if you up the brightness on the laptop, or use a 1080p screen instead of a 768p one. Anandtech did a review on an acer laptop with a ULV i5 and a 640m and they were cpu bottlenecked a few times. Note that a ULV i5 is only 10-20% slower than a i5 standard voltage and the 650m is about 10-30% faster than the 640m