The main differences between 18% nickel frets, 25% nickel frets, and stainless steel frets lie in their composition, durability, feel, and maintenance requirements. Each type of fret material has its own unique characteristics that affect their performance on the guitar.

Typically American-made guitars use 18% nickel as the standard but some cheaper guitars use 14% nickel which would wear out very quickly for most players. Another thing to consider is that the type of string you use will impact the wear on your frets. For example, D'addario NYXL's will cause more wear than a typical EXL or Ernie Ball string. So there are many factors to consider.

Here's a detailed breakdown:

1. Composition

1. 18% Nickel Fretwire: This is typically a Nickel-Silver alloy (sometimes referred to as nickel-plated brass) containing 18% nickel, with the remainder mostly being copper and zinc. It is a common, standard material used in most guitars.

2. 25% Nickel Fretwire: This also uses a Nickel-Silver alloy, but with 25% nickel content, which makes it slightly harder and more resistant to wear compared to 18% nickel. The rest of the alloy is mainly copper and zinc.

3. Stainless Steel Fretwire: Stainless steel is a different material altogether, composed of chromium, nickel, and iron (with typically 12-15% chromium and 8-12% nickel). Stainless steel frets are much harder and more resistant to corrosion compared to nickel-based alloys.

2. Durability and Wear Resistance

1. 18% Nickel Fretwire: This material is relatively soft compared to the other two options. While it offers a decent lifespan, it is prone to wear more quickly, especially with heavy playing, bending, or aggressive use of the fretboard. Players who use a lot of vibrato or string bending may notice significant fret wear after a couple of years, especially on the higher frets.

2. 25% Nickel Fretwire: These frets are more durable than 18% nickel frets due to the higher nickel content, which increases hardness. They still wear over time, but they last longer and are more resistant to corrosion and tarnish. However, they still aren’t as hard as stainless steel.

3. Stainless Steel Fretwire: Stainless steel frets are significantly harder and more durable than both 18% and 25% nickel frets. They resist wear and tarnish exceptionally well, often lasting 2–3 times longer than nickel alloy frets, even with heavy use. This makes them ideal for players who want frets that will last many years without significant wear, even under intense playing conditions.

3. Feel and Playability

1. 18% Nickel Fretwire: Frets made from 18% nickel are softer and offer a warmer, vintage-like feel. They’re easier to crown and shape, which makes them easier to work with during setup or fret leveling. Many players find the feel smooth and familiar, especially those who are used to older, more traditional fretboards. However, due to the softness, you might notice some fret wear after extended use.

2. 25% Nickel Fretwire: Frets made with 25% nickel are slightly firmer and harder than 18% nickel, giving a more responsive feel. They are more durable, but the increased hardness might make them a little more challenging to work with when it comes to crowning or leveling. The feel is still relatively smooth, but slightly more "solid" compared to the softer 18% nickel.

3. Stainless Steel Fretwire: Stainless steel has a slicker, more slippery feel due to its hardness and smoothness. The frets require a bit more work when it comes to setup or fret leveling because of the material's toughness. However, many players find the smooth feel of stainless steel frets beneficial for fast playing and bending, as there is less friction between the string and the fret.

4. Maintenance

1. 18% Nickel Fretwire: Nickel-based frets (18% or 25%) require regular maintenance, especially for players with a lot of bending or aggressive play. Dressing, crowning, and polishing are often needed every few years to keep them smooth and to prevent wear spots. They may also tarnish or corrode more easily, especially if the guitar is not stored properly.

2. 25% Nickel Fretwire: These frets last longer and require less frequent maintenance than 18% nickel frets, but they still need attention over time. They resist tarnish and corrosion better but will still eventually require fretwork as they wear down.

3. Stainless Steel Fretwire: Stainless steel frets require the least maintenance because they are highly resistant to wear, tarnishing, and corrosion. They hold up extremely well, and polishing is needed much less frequently. However, because they are harder than nickel-based frets, they can be more difficult to level or crown during setup and re-fretting.

5. Cost

1. 18% Nickel Fretwire: Typically, less expensive than both 25% nickel and stainless steel frets. It is a more common material used in entry-level to mid-range guitars.

2. 25% Nickel Fretwire: Moderately priced, more expensive than 18% nickel but still less expensive than stainless steel. It offers a good balance between durability and cost.

3. Stainless Steel Fretwire: Stainless steel frets are generally the most expensive option due to the hardness of the material and the difficulty of working with it. They are often found in higher-end or custom-made guitars.

Summary:

18% Nickel Frets:

1. Pros: Softer, warmer tone, easy to work with, affordable.

2. Cons: Prone to wear, requires more maintenance over time.

25% Nickel Frets:

1. Pros: Longer-lasting than 18% nickel, more durable, slightly brighter tone.

2. Cons: Harder to work with than 18% nickel, but still softer than stainless steel.

Stainless Steel Frets:

1. Pros: Extremely durable, long-lasting, brighter and clearer tone, minimal maintenance.

2. Cons: Expensive, harder to work with during setup, may feel slicker or different to some players.
    

In conclusion, the choice between these materials comes down to how much you prioritize durability, feel, and maintenance. If you want frets that last the longest with minimal upkeep, stainless steel is the best option. If you prefer a warmer feel and tone with a traditional vibe, 18% nickel may be more suited to your preferences. 25% nickel is a middle ground, offering better durability and tone clarity than 18% nickel, but without the premium cost of stainless steel.