Classical guitars, because of their nylon strings, have some unique requirements for frets. The same frets that will work perfectly well for, say, steel string acoustics will not do well on a classical guitar. The primary reason for this is the elasticity of nylon. With frets of a given height, it's much easier to press a classical guitar string farther toward the fingerboard than it is for a steel string. If fret crowns are too low, the finger fretting the string may reach the fingerboard surface and be stopped from pressing further before enough string-to-fret-crown pressure has been applied to prevent buzzing. In short, classical guitar frets need to be somewhat higher than steel string frets.
A recent development in fretwire is stainless steel. Stainless fretwire is ca 50% harder than nickel-silver and will probably last as long as the life of a classical guitar, which wears out frets much more slowly than a steel string guitar. Stainless frets entail an added-cost charge because the fretwire is very stiff and hard: this requires custom bending of each individual fret to precisely fit the shape of the fingerboard to make sure they do not come loose later; the fretwire itself also wears out cutting and shaping tools much faster than nickel-silver.
Should you get nickel-silver or stainless frets? I'd say if your guitar were steel string, go stainless. With a classical guitar, the argument is less compelling. My experience has been that nickel-silver frets on a classical guitar typically need fret re-dressing every 5-7 years for players who use their instruments for several hours daily. One re-dressing is likely all that the such frets will endure; a second re-dressing will probably make some of the crowns so low that the guitar's playability will be affected. By the time the guitar has gone through one fret re-dressing and then later needs refretting, it's likely the fingerboard configuration will benefit from some tweaking anyway. But on the other hand, the extra durability of stainless frets means your guitar will have perfect fret crowns much longer.
I offer several different fret options on my guitars—
Medium frets offer a very good playing feel for most guitarists. Fretwire of the size used for these frets is available for purchase in nickel-silver, but I do not recommend it. The reason is that one re-dressing to remove worn places may reduce the height of some of the frets (viz. f1-f5) to the point where playability could be affected for some guitarists. This is not a problem with stainless steel because you will probably never need to have the frets re-dressed. Dressing my newly installed frets typically removes only .001"-.002", which is less than is usually required, because my epoxy-casting installation strategy produces very even crowns to begin with.
High frets are the default on my guitars. They offer a very good playing feel for most guitarists. These frets can be re-dressed one time to remove worn places and still retain enough height for a classical guitar. You can expect at least a decade of use from these frets before refretting is needed. Fretwire of the size used for these frets is not available for purchase in stainless steel.
Highest frets are sometimes preferred by guitarists who concertize professionally and/or whose playing style gets very vigorous. The additional fret height allows the fretting finger to push harder on the string before reaching the fingerboard surface. On the other hand, some guitarists accustomed to normal frets may find the Highest frets somewhat awkward.