New fret on my guitar doesn't look new...

I'm not a luthier, but it looks like your person may have installed new fret(s), and then filed them down a bit to reduce "buzzing" which can happen when a string makes contact with a fret that is too high. This procedure is called "fret dressing". I believe that it's part of a new fret setup for a guitar, along with adjusting the truss rod. As guitarists, we want the action as low as possible (minimal relief in neck) but minimal fret buzz (their height relative to one another). It's a fine balance between one and the other. Here's a couple of good article on frets:

http://avhguitarrepair.com/repair-blog/ … ret-dress/

http://www.fretnotguitarrepair.com/repa … r/buzz.php

Generally speaking, I believe it's a safer idea to replace all of the frets from the 5th position to the nut simultaneously. These are the 5 that get the most wear usually, so replacing them all will minimize the setup problems. I hope this bit of info is useful.

Yeah Curt, sounds like your Luthier did not finish the job.  That fret still needs to be crowned.

As for your Mexican Strat...... they have a reputation of being fine instruments.  Usually of better sound & quality than some of the USA models in the same price range.  I'd consider keeping it until you are able to be sure that the one you have to replace it is worth keeping.

I have a mexican strat and I love it,it sounds and playes as well as any stateside strat.

Yep!

But the sound of my strat is less good since I changed the frets. (The old frets were "rounded", I mean, the shape was the basic shape of a fret, but they were very worn)

I'll try both guitar (Mexican/USA) at the same time, to have a better point of view ^^

Thanks to all for your answers!

You can physically measure the strings using a vernier caliper of the digital kind.  Just remember to choose inches for the purpose, although they will also read out in mm.   If you can actually "see" the difference between .009 and .010 .... your eyes are much better calibrated than mine, it's only a thousandth  of an inch.

Both would be considered Light or Extra Light strings.... and "bend" really easily.  You can get too thin as well as too thick.  For example the "nut" is cut to a given width for the string "guage" (with a little room of +/- a couple thousandths) and if you go too small, the string can actually "rattle" in the gap sounding very much like fret buzz.  Particularly noticeable on necks without a "zero fret".   Could be a bit of that "metallic" sound you describe.