Enlarger lenses are generally grouped into three price categories. I refer to them as: amateur lenses (very cheap), regular "professional" lenses (moderately priced), and apochromatic (APO) lenses (very expensive).
Amateur lenses are usually already mounted on the enlarger when you purchase inexpensive, small format enlargers. Frequently, such inexpensive enlargers cannot accept other focal length lenses.

Professional lenses are usually sold separately from the enlarger. You select the focal length and appropriate lensboard that you want for a specific type of work.

APO lenses are available in only a few different focal lengths. They are commonly used for making very large magnification prints and some high quality copy work.

There are three major companies that are currently making enlarger lenses: Rodenstock, Schneider, and Nikon. All three make lenses in several different price ranges.

Rodenstock's model, called Rodagon-G, is probably the most expensive. Think of them as being sort of like a Cadillac. Regular Rodagon lenses are a little less expensive and more commonly used for everyday professional darkroom work. Rodenstock also makes a model called the Rodagon-S, which is less expensive than the regular Rodagon and is also very popular among professionals. The Rodagon-S is probably the least expensive lens made for professional use. I've used a Rodagon-S 50mm lens for years. I'd hate to have to count the number of wedding prints that I've made with it.

Nikon's brand called El Nikkor lenses are priced more like Chevrolet products. They are a little more expensive than the Rodagon-S lenses. El Nikkor lenses are good lenses. I've used an El Nikkor 75mm lens for years and love it.

Both Rodenstock and Schneider make some wide angle lenses. Rodenstock's are called Rodagon-WA lenses. Schneider's brand is called Componon-WA lenses. Both brands are a bit expensive. They are used when you want to make a relatively large size print without having to raise the head of the enlarger as high as you might otherwise have to. With a wide angle lens you can also use a larger negative format and work with the head lower toward the easel.

For example, while a regular 50mm lens cannot be used with negatives larger than 35mm, a wide angle 50mm lens can be used with negatives larger than 35mm. Wide angle lenses are not commonly used by amateurs due to their greater expense.

Schneider's brand called Componon-S can be thought of as being priced in the mid range, a little like a Buick or Oldsmobile. They are a bit more expensive than the Rodagon-S series of lenses.

Both Rodenstock and Schneider make APO lenses. APO lenses are simply very expensive. El Nikkor does not offer an APO lens.

The focal length of a lens must be equal to, or greater than, the diagonal of the film format when measured in millimeters. A 4x5 negative will require a lens of at least 135mm focal length. If you are going to be using a medium format camera and shooting 6x7cm format, you will need an enlarger lens of 90mm (or longer) to print those images. Negatives of 6x6cm or 6x4.5cm require a lens that is at least 75mm long. If you are going to use a 35mm camera, you'll need an enlarger lens of 50mm (or longer) to print that format.

Yes, you can use a 90mm enlarger lens to print 35mm negatives, but because of its longer focal length, you will have to have the head of the enlarger raised much higher in order to make the same size print that a 50mm enlarger lens would make with the head much lower. With a very long lens it is possible that you might not be able to raise the enlarger head high enough to make the size of print that you might want to make. Thus, most folks use the shortest lens that will work with a given negative format, allowing them to make the largest possible print with their particular enlarger.

If you only have a 90mm enlarger lens, it is possible that you might not be able to make wallet-size prints (less than 1:1 size ratio) from your 6x7cm negatives because the head of the enlarger might not be able to be moved low enough to accommodate the 90mm lens. You might need a 135mm lens in order to make wallet-size prints from 6x7cm negatives. It all depends on your specific enlarger and how low toward the easel you can move the negative plane.

A 90mm lens would commonly be used to make wallet-size prints from 35mm negatives for the same reasons. Most enlargers will not allow the head to be lowered enough for a 50mm lens to produce wallet-size images from 35mm negatives.

If you think that you might ever want to blow up a small portion of a negative, you might want a very short focal length lens like a 25mm. But, remember, such a short lens can only be used on a very small portion of a negative. It cannot "see" an area much larger than the size of a disc negative (smaller than a 110 negative). Such a short lens will require a recessed lensboard.

You might want to use an extended lensboard with your longer focal length lenses like the 90mm or the 135mm. An extended lensboard (while not absolutely necessary for routine work) can be helpful when you are trying to make very small prints or prints that are actually smaller than the negative (less than 1:1). Use a regular, flat lensboard with a 50mm lens.

Beseler offers a line of "professional" lenses that are made for them by Rodenstock in Germany. They are called Beseler HD Lenses. They are priced just a little more than the Rodagon lenses and a lot less than the Rodagon-G lenses.

If you set two different focal length lenses at the same f/stop, then whatever enlarger exposure settings that work with the first lens will produce the same exact image with the second lens, provided that the second lens is adjusted to produce the same exact print size as the first lens.

It doesn't matter that the enlarger head might be several inches higher or lower with one lens than the other. The only thing that matters is that the size of the projected images be the same and the f/stop must be the same in both cases. There might be a slight color difference between the glass in the two lenses--maybe 2-3cc. It can be ignored with most, noncritical images.

A "long lens" will have to be farther from the baseboard to produce the same size print that a "short lens" can produce when it is closer to the baseboard. But, in both cases the different lenses would be producing the same size print. Both prints will require the same, exact exposure settings (f/stop and time).