For Harry Berv in A Creative Approach to the French Horn the double horn was the horn of choice for students at all levels.
In school systems, single F and Bb horns are available for students who do not intend to pursue performance on the horn as a career. But the student who intends to major in horn should start on a good instrument.
I strongly recomment [sic] the serious student purchase a double horn in F and Bb. If you purchase a single F or Bb horn, when the time comes to switch to the double horn, you will have to spend more money and learn a new fingering on the double horn. Purchasing a double horn at the outset gives a giant stride in your immediate general progress and future overall performance.
On testing horns, and the topic of “resistance”
Berv has suggestions as to what to test on a horn, with tone, intonation, and resistance being major factors. About the latter he notes
When you blow through the horn, there should not be too much resistance. All registers—high, middle, and low—should be about equally resistant, so that you do not have to exert undue pressure on the embouchure, especially in the high register of the horn. If too much exertion is demanded in the upper register, it can cause the embouchure muscles to become strained and stiff, and they will not be able to respond quickly enough for the instant reflex changes required to cope with the horn’s range.
Resistance is an interesting topic. Some horns feel relatively stuffy and some relatively free, but your specific perception of either relates to the resistance of the horn you are most used to.
We know what resistance feels like, but what causes it? Weight of the instrument and the bracing pattern is part of it, as is the bore of the tubing, leadpipe taper, and the bell throat. Tight bends in the tubing perhaps as well. Beyond those elements, be aware also that a perception of resistance may be related more to the mouthpiece fit. Many elements contribute and all impact your ability to play well and with good endurance.
The horn as an extension of your physical body
He is also concerned with finding a horn that is balanced well, fits the left hand, and puts the mouthpiece out at a comfortable angle. Berv concludes,
In my many years of playing, I have always tried to find a better horn, but many times I have had to make do with the horns available. In the final analysis, the problems on the horn can be overcome with patience, hard practice, a search for compensation, and sensitivity. The chosen horn must fit the individual’s taste in the end result. No matter how good a new horn or mouthpiece feels at first, it takes time to feel completely secure and at home on it. The horn is an extension of the player’s physical body and must be treated and played as such.