What are your degrees in both of your eyes? I have +2.0 in both of my eyes. So usually I will choose the standard plastic lenses. But if you have high prescriptions, the 1.6 lenses are good for you because the edge of this kind of lenses will be thinner and they are very light so that they will not put heavy burden on your nose.
1.5 lenses are the standard lenses while higher index such as 1.6 works best for high prescriptions because this kind of lenses are thinner, lighter and can make people feel more comfortable. You can have a look at http://wiki.answers.com/Q/How_does_one_determine_which_high_index_lens_is_best_for_their_prescription_when_every_optician_gives_a_different_answer#slide14
1.6 lenses are suggested for prescriptions of +/- 3.00. It is said that 1.6 lenses are 25% thinner than standard plastic lenses so that they are ideal for higher prescriptions. Compared with the 1.5 lenses, 1.6 lenses are thinner and they can work well with thinner metal frames so they will look better. Besides, they are very light.
It depends. Generally, the 1.60 index lenses are thinner than 1.50 index lenses. That is to say, if you have strong prescription, 1.60 index lenses is highly suggested. However, if you have low prescription, 1.50 index lenses are enough. But if you have strong prescription and use 1.50 index lenses, the lenses look much thicker than 1.60 index lenses. Hope helpful!