The most important consideration when deciding whether to go with a skin mount or repro is which artist will do the work. Either type of mount can be very good or very bad. Done right, there's not alot of difference visible to the eye.
A fiberglass repro will last simply because it's fiberglass. A skin mount that's done RIGHT (that's the key) will last too. We've all seen shoddy skin mounts that are yellowing (improperly degreased), have raised scales (poor mounting techniques), or have unnatural shrunken areas (poor finishing techniques). ALL the top fish taxidermists will use cast heads on all coldwater fish. DON'T accept the use of the real head. If the taxidermist you are considering insists on using the real head, find another taxidermist. That's not just my opinion, it's the industry-wide standard. Real heads will shrink and must be built up with epoxies, furthermore, it's not possible to completely remove all grease and oils which will eventually bleed through the finish. To see this for yourself, look on the underside of the head (in the throat area) of virtually any salmon or steelhead mount which has the real head and has been around for a few years. Chances are you'll see what I'm talking about.
Some info about repros; the taxidermist usually does not make a mold of your individual specimen (not possible if you released it), instead a "blank" is used that matches your measurements as closely as possible. Sometimes a custom mold can be made of your fish, if you have it, but expect to pay more in most cases. In order to be able to cast a "blank" of your dimensions, at some point a mold had to have been made of an actual fish the same size as yours. If you do someday catch a 50 lb steelhead it's going to cost a pretty penny to have a repro done because a fish of that size has never been killed and molded. An existing blank could be modified, which is difficult and very time consuming (expensive) or a carving could be made then molded to produce your repro (more expensive).
Commercially produced repro blanks vary greatly when it comes to quality. Some are very good but even more are junk. Look for yourself at the taxidermist's repros and see if they look natural and lifelike. I'm not talking paint here, but anatomy and pose.
Painting repros is a struggle for some taxidermists because there are absolutely no natural markings to start with. This is where we really separate the good from the not so good. Each artist has his own style and the results vary but the bottom line is- does it look like a steelhead? (or salmon, or whatever) Sounds simple, I know, but unfortunately it isn't. Anyone can spray on some airbrush paint but the best guys can make them look alive.
Prices vary alot too. The reason the best are more expensive is the amount of time put into each mount. Many more hours are required to do the job right and not cut corners. Of course, putting all that fine detail into the paint job takes more hours too. Don't ever forget, you plan to have that mount on the wall for decades. Saving a few bucks now could cost you in the future if the finished product is of poor quality. Price is always a consideration of course but if at all possible it shouldn't be the only reason for choosing one taxidermist over another. Quality is the most important factor.
Sorry to make this post so long, but I HATE to see a trophy fish that ends up looking like a piece of junk hanging on someone's wall. It doesn't have to be that way.
Take a look at the photos on the page below. Each photo is labled whether it's a skin or repro but try to disregard this. You decide if repros look unnatural.
http://www.twinriverstaxidermy.com/photo_gallery.htm