Telling musicians to evaluate and choose their instruments in a “scientific” way?

Exactly right and important to keep in mind that artistry on a high level requires an instrument of similarly high quality.

As has been pointed out, ultimately what matters most is the player’s musicianship that matters most. However, this is really stating the obvious and the differences and the practically infinite nuances that exist in the playability of different instruments are very important to an accomplished player. Ultimately, those differences are what allow a great player to take his playing one step closer to the sound and expressivity that he is striving for. An inferior, or unsuitable instrument simply won’t allow a player to get there. Great players can make good music on mediocre instruments, but there is a reason that they don’t play mediocre instruments.

It becomes a very personal choice. Some players want an instrument that is very easy to play in the sense that it responds very quickly and with little resistance. Other players prefer to have an instrument that offers a little “fight” and needs more coaxing to respond. The particular playing situation may determine in part the choice of instrument. The quality of the tone is a result of an inherent quality in the instrument and of the particular player’s touch, which is the result of the player’s individual approach to the mechanics of producing a sound as well as his physical characteristics.  An accomplished player knows how to control and manipulate these in order to produce the desired sound.  

+1 tommylion

Frogman explores the issue far more eloquently than I ever could. And oh yeah... A few years back I happened to score a box of genuine tortoise shell guitar picks that had been sitting around unsold since they were still legal.  Yes, I am glad that the tortoise is no longer hunted down for its shell,  But the fact remains that not only do they truly make my steel string acoustic guitars sound better than a plastic one ever did, they reshape themselves over time to make themselves ever more comfortable to hold. 

Excellent analogy. 
Further, Gibson players don't denigrate Fender players for having inferior equipment. Effects pedal makers aren't accused of using snake oil because someone says they can't hear the effect, etc.

I'm not a guitar player but have dittled with some at the music store.  I can Feel the difference just holding and moving my hand upon one guitar from another.  Watching guitarist friends trying out different guitars.  Sound is first off.  Listening while strumming chords.  Tune it up.  But once past that they seem to be more interested in moving upon it to make music.  Kind of showoffy really.  Finger acrobatics. I can't imagine how objective science via an oscilloscope will show this.

This goes beyond guitars.  Piano, violin, clarinet...  Stereo system.  Years ago I went out to purchase a new french horn.  That horn of all horns for me. Got down to 4 manufactures/models costing between $8-12,000 at the time.  They all are fine horns as resale values represent.  Still fine horns by todays standards.  The one I purchased ultimately was because it was the most Friendly to me.  Did what I asked it to do.  Almost helping me play my best.  Objective scientific measure?  Not yet.