As far as saddles go, its the same theme as seatpost height--you never know if you have enough until you have too much (Paraphrased from Sheldon Brown). When the saddle is too wide, this is obvious, because that hinders circulation, hinders movement, or causes hip pain. When the saddle is too narrow, that is insidious, because it causes pain that limits distance of cycling, and occuring in proportion to the distance.
Saddles do come in sizes and you can measure yourself to see what size you need. The Specialized bicycle dealer has a nifty gimmick for this. Prior to that, the method was to place a pie shell between two pieces of thin paper (or wax paper) and set that onto a footlocker or ice chest. Sitting on either bit of measuring equipment will cause two dents to appear on the surface. This is known as the sit bones measure. Using a metric ruler, record the distince between the centrepoint of the two dents. That is your saddle size. One may use a slightly larger size if the saddle seems overly flexable. The curve or lack of curve at the saddle's "edges" do affect its usable size. Therefore the measurements provide only a basic clue rather than an exact determination.
Too soft and too hard have the same result, and that is "hot spots" that develop into contusions. To avoid the contusions, its usually considered best to vary one's seating slightly during the bicycle ride. Standing up for the upper part of hill climbing and/or moving forwards or back to suit cadence, are partially helpful. Carefully adjusting the saddle angle may also help. A pitched forward saddle may harm the spine because of sliding, so correct saddle angle is also non-slippery. A "Two Bolt" seatpost, like Thompson, or a "cam action" seatpost, like Salsa, can make more precise adjustments for angle. When all else fails, select either a harder or softer saddle and try again. Many cyclists do have a collection of saddles; therefore, consulting the local bicycle club can be helpful.
To answer the question of which saddle is best. . . Unfortunately, the only answer is: "The one that works." There are a few clues to help: Circulation, Distance, Hill-climb speed, and "sit bone" spacing.
Saddles and shoe attachment: A sportive person may apply twice or several times their own body weight, unto the saddle. This happens during sportive riding when shoe attachment is actually utilized. Most recreational saddles will "break over" the edges (becomes very narrow during use) or "sink down" in the middle (slides the cyclist forward onto the horn) under these conditions. Therefore, selection of an appropriately durable, "sport centric," saddle may be an important consideration, in order to maintain the correct size / fit during active use.
- Bicycle Seats Explained - An article by Jim Langley
Jim really hits the nail on the head. I would add that more cushioning or softer materials is seldom the answer for seat pains. Probably the softest saddle you can ride any significant distance, without chaffing, is leather. You will often find distance cyclists with tens of thousands of miles on their leather saddles. I have over thirty thousand on my own, and it is no where near ready for replacement.
Leather saddles aren't as hard to break-in as many have led us to believe. There may be better products, but I've always used mink oil. I loosen the leather, saturate it and rub it inside and out 'til it is soft. I ride it slightly loose, for a while, just making it good and soft.
Some recommend beating a leather saddle, with a bat, to break it in. I believe this damages the grain of the hide. Then, you get weak spots and it wears out too soon. Others ride with their hide too loose. This too can damage the grain and can make it blouse out, causing chaffing, like an overly padded saddle. Hand rubbing and flexing a leather saddle takes a bit longer, but the result is perhaps the most comfortable saddle you will ever ride.