One form is not preferred over another. It is more important to have a reputable company making it so it is consistent from batch to batch. But below came from a blog member in April 2014:
I normally recommend drops as they are the cheapest by a mile here in Ireland. I religiously check the blood levels after two months. About 20% of my clients have a poor response to the drops. When this happens I switch them to a sublingual spray which very quickly gets their levels up. It is a pity they did not test this dosage form.
Pharmacist, Naturopath, Herbalist.