Like Tweake says, it is highly debated although there has been a great deal of research done lately. I have had a great deal of interest in this question. It basically comes down to personal choice.
For a thousand years cork was the only choice and for good reason. It is natures perfect seal. The only problem was the risk of cork taint (
http://en.wikipedia.org/wiki/Cork_taint) which with modern techniques of making corks has been practically eliminated.
Cork taint is the main reason for the development of other types of closure. Only problem is that no matter how they improve crown caps, screw caps or synthetic cork it has never matched natural cork. Then there is the environmental impact. The carbon footprint of cork is tiny compared to crown/screw caps or synthetic. See the attached image.
This lengthy report goes into a lot of boring detail.
http://www.corkfacts.com/pdffiles/amorim_lca_final_report.pdf
Most wines/meads never make it past 24 months before they are drunk so cellaring is not an issue. Any closure will suffice. I'd be willing to bet duct tape would get the job done. This question really has no bearing unless you are a commercial winery/meadery. Only then will you consider specific cost and marketing factors.
A lot of wineries are going to screw cap because of the cost. I personally think this is actually a foolish move and environmentally irresponsible. This report backs me up.
http://www.tragon.com/downloads/news/18.pdf
Too illustrate my point, Hogue Cellars makes some great wine. A few years ago they decided to go to screw cap. I will never buy another Hogue Cellars wine. It tells me they care more about the bottom line than the quality of their wine.
Anyway, sorry if that went a bit astray. My point is for the homebrewer the choice of crown cap or cork will have no impact on your mead. Enjoy your new hobby.