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.
They compared the effects of different forms of vitamin D supplements on blood levels of 25(OH)D and concluded: “Oil-emulsified vitamin D3 supplements resulted in a greater mean change in serum 25(OH)D concentration.”
I personally use Biotech vitamin D3 capsules. They are competitively priced and I have no problems maintaining 25(OH)D levels in the 50-60 ng/mL range when taking 3,000 or 4,000 IU daily.