The key here is whether your child (or whoever is playing) is a beginner, a casual player, just trying the instrument out, or is really into it and is seriously expressing herself/ himself through the instrument. You should rent an instrument for the beginner/ casual player and buy one for the serious player.
There. Don't you wish all of life were that easy?
The question quickly becomes, when do you buy that instrument. Not only is an instrument a major investment, it is also an extension of its owner's personality, and it needs to fit that person. You need to know what kind of music your child really wants to play, and in what situations. You may be insisting on classical lessons, but if the kid really wants to rock out with his garage band, then an electric guitar is more appropriate than an acoustic one. A marching band clarinet is of different quality than a concert hall clarinet. All violins may seem the same, but they don't feel that way to the musician. So when you're buying an instrument, the family piano aside, your child has to be old enough and musically mature enough to give you meaningful input about it. If the question “Do you like this one?” produces a half-hearted “Yeah, I guess,” then the time isn't right to buy. Violins and cellos come in a range of sizes like quarter and half-sized, for the small musician. You most likely don’t want to purchase those until your child is finished growing.
As your child develops more advanced musical skills, they will be held back and frustrated by a poor quality instrument. If it is time to graduate from a student-model, beginner instrument to one with finer features and craftsmanship, it might be time to buy such an ‘upgrade’ for your child.
Some serious instrumentalists, particularly those in school bands, may want to try out a second instrument to their primary one. Woodwind players often play multiple instruments; a clarinetist will also play some saxophone, and maybe the flute as well. In the case of playing numerous instruments, you would probably want to rent the secondary one or borrow it from the school.
There are rental terms that you must be aware of. As with any rental, you bust it, you buy it. Some instruments have seen tremendously hard wear from renters, so make sure you inspect an instrument carefully and have all its nicks and dings recorded before you take possession of it, especially if you're renting-to-own. Read the terms of your contract carefully and ask around for referrals to rental shops with reasonable terms. If you get a bum instrument, return or take it back for repairs as soon as possible.
Rental options begin with month-to-month, which is generally the most recommendable one. If your kid is in band, then a rental agreement for the school year or semester is also good. If you choose rent-to-own, you should make sure that 100 percent of the rental fees are credited toward the purchase price.