Most evidence I have seen indicates that vitamin D3 is more potent than vitamin D2 as a supplement.
Reinhold Vieth, a well known vitamin D researcher produced a review article on this subject back in 2006 detailing the evidence for D3 vs D2 at that time.
The case against ergocalciferol (vitamin D2) as a vitamin supplement.
Some of the individual studies include:
Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2.
This study, also by Vieth, found D3 showed 1.7-times greater efficacy than D2.
Vitamin D2 Is Much Less Effective than Vitamin D3 in Humans.
This study found: that: "Vitamin D2 potency is less than one third that of vitamin D3. Physicians resorting to use of vitamin D2 should be aware of its markedly lower potency and shorter duration of action relative to vitamin D3."
Short and Long-Term Variations in Serum Calciotropic Hormones after a Single Very Large Dose of Ergocalciferol (Vitamin D2) or Cholecalciferol (Vitamin D3) in the Elderly
In this study elderly patients were given a single dose of 300,000 iu of vitamin d. They concluded "Cholecalciferol is almost twice as potent as ergocalciferol in increasing serum 25(OH)D, when administered either by mouth or im."
Vitamin D3 Is More Potent Than Vitamin D2 in Humans.
This study gave 50,000 iu once a week of either D2 or D3. They conclude: "D3 is approximately 87% more potent in raising and maintaining serum 25(OH)D concentrations and produces 2- to 3-fold greater storage of vitamin D than does equimolar D2."
I only know of one study by Michael Holick, another well known researcher on vitamin D who has written a book on the subject, that found that both the D3 and D2 forms were equivalent. However he only used a dose of 1000 iu per day and at this dose differences between the two forms my not show up. There appears to be some dissagrement between Holick and others working on the subject. I find it strange that Holicks papers on the subject appear to ignore almost all of the other studies showing that D2 is less effective. Vitamin D2 Is as Effective as Vitamin D3 in Maintaining Circulating Concentrations of 25-Hydroxyvitamin D.
Vitamin D2 has been the form used as a prescription medication for a long time. D2 has also been the form used to fortify foods like margarine and milk for many years. There is a great deal of inertia behind such practices and the pharmaceutical companies that produce vitamin D2 have little incentive to change things. Doctors are often limited in which form they can prescribe.
Vitamin D2 does work, but it appears not as well as the same dose of D3. Vitamin D3 supplements are not expensive now and are available to most places. If you want you can keep taking the D2, see if they work and when they have corrected the levels switch to your own supply. Or just buy your own now.