It really depends what you prefer I personally use the Mee audio x7 plus, they have amazing sound quality and are comfortable and a decent price as well. I know you said your budget is $40 but I would try to increase that a little and you can get a lot more quality for your money, I heard a lot of great things about the Jaybird X3's they are worth considering but they are 80-90 I think
Here's a good resource for wireless earbuds which should help - http://www.flipflopsandapplesauce.com/best-bluetooth-earbuds/