Ciao Claudia,
There are a number of multi-level marketing essential oils for theraputic purposes. I cannot say I have tried aroma-zone oils, but I can agree with lisa on the fact that Young Living going to great lengths to test and source quality essential oils.
If your serious about the quality of oils try asking the supplier some of these questions, the plant and country origin, distillation method, any dillution of the oils, your the oils supplied directly from the distiller, what kinds of qualitity standards they meet (ISO, Certified Organic), the harvest to distillation time of the herb to oil and the age of the product. Oilganc has a essential crop harvest calender at http://www.oilganic.com/essential-oil-crop-calendar-1armond.htm.
Also take into account the size your ordering and the personal feelings you have about the supplier and if they click with you.
Cheerio,
Vito