Which combination has a lower gear friction?
According to Sariel's gear tutorial in chapter 5, Efficiency:, 

- the less gears, the better
 - the smaller gears, the better

Based on this principle the first setup looks the best, but you are right that the 8 tooth gear is the weakest of all and it has the most backlash too, if any of these are valid concerns for your build.

Note that gears can be strengthened by using them in parallel by putting some 24 tooth on one axle and the same number of 8 tooth on the other, mating them together all at once.

If you are concerned by backlash, you can experiment with springs, rubber bands or other ways to put a constant load on the system.