Just buy trays that are a couple inches deep so that the plants have room to develop good root systems if you are going to grow sprouts on soil, otherwise you can just grow most sprouts in jars. The instructions on how to do that are on the first page of this thread.
Please, forgive me for asking something that was probably discussed already, but the thread is so long... I want to buy trays asap but do not know which trays to go with because they are different for water growing and for soil growing.
Growing sprouts in soil is actually better, because the sprouts can pick up nutrients from the soil, despite the fact that many people say otherwise. I can feel a slight difference between eating soil-less sprouts and sprouts grown in soil, too - the ones grown with soil are always better. This is especially true for sunflower greens, they seem to be able to pick up even more nutrients from the soil then other microgreens. So you should grow them on soil.
If somebody just can tell me shortly, what the difference between growing sprouts on soil and on water. I've just read a document about various studies made on sprouts and their nutritional value and I got an impression that growing on water is way better.