Yasmin is widly know for causing less weight gain than other birth control brands, but one never knows. If you're very thin you shouldn't really worry about gaining weight. Besides, would your rather gain weight then get pregnant? I sure would.
If you usually get sick on birth control i would suggest a low does monophasic pill. Like Mircette, or Alesse. The lower estrogen, the less side effect you'll *probably* have.
Also, i've never heard anything about Diane 35 being less effective then other birth control pills. If it WAS less effective the FDA would have pilled it off the market ASAP. All birth control pills are equally as effective, no matter what the dose of hormones is. Low does pills and high does pills protect you JUST the same, as long as you use them properly.
Good luck