First off, visor masks are only needed for FIE events.
The FIE maintains a list of approved masks (
click here). This is a list of manufacturers - sometimes vendors will buy an approved mask from the manufacturer and put their own label on it as well. So long as a mask is from one of the approved manufacturers it is probably as safe as one of these things can be however that is not to say that the safety concerns can be completely ignored. The FIE medical committee has expressed concerns regarding air circulation within the mask and the FIE SEMI committee has expressed concern regarding safety, both of which were ignored by the FIE executive committee (although the recent decission to temporarily suspend the requirement for use of visor masks at FIE foil events pending investigation of a recent incident may indicate some second thoughts on their part).
Much of the concern regarding safety isn't based so much on the condition of the mask when it leaves the factory as what it will be at some point down the road. The lexan faceplate has an expiration date and should be replaced every two years. Replacing the faceplate requires partially disassembling then reassembling the frame which holds it in place (lots of nuts to remove and put back). Given the inability of some people to perform even the most simple, basic maintenance this is probably asking for trouble, especially for a mask which isn't regularly inspected.
As far as individual features of different masks are concerned, removable bibs make is easier to replace a bib which fails resistance testing (generally the first thing to fail on any saber mask). A seperate "scratch" layer provides an relatively inexpensive sacraficial layer that helps to protect the more expensive lexan underneath from scratches, thereby extending its useful life (within the two years previously mentioned). Beyond that it pretty much comes down to fit and feel (people tend to have different head shapes and almost some masks will fit better than others).
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