When I was still a young Scout, my Troop formed a high adventure patrol. This was before Venturing, and we were on a Air Force base 30 miles from the closet city. I did not have the gear for backcountry exploration, so my parents got me a external frame backpack for Christmas. I should point out, I did try the pack on at the store, and my dad did have me walk around the store with it on, etc.
On my first backpacking trek, I was miserable and even cried through a lot of it. The pack just slid down me and rode on my shoulders, my legs where just a bundle of cramps and my shoulders were on fire. My dad, who was a leader in the Troop, stayed back with me and helped me through it. I did make it to the camp and was so proud of myself.
It turns out that trip was like 12 miles each way, but our leaders thought it was only a few miles. It also did not help that the pack did not fit or ride on me properly. I never used that pack again for backpacking. However, I did keep going in to the backcountry. Over then next few treks my dad rented several different packs for me. We soon found that a simple internal frame, almost like a duffel bag with straps worked best for me.
When my birthday came around that summer, they got me a new internal frame backpack that I still use to this day. The moral of this story is that often times the only way to know if a certain piece of gear will work as expected for you is to try. Now lets face who can afford to keep buying gear till they find the right one, and how many stores will return used gear?
What is the answer, do what my dad did with me, simply rent it first to ensure it works for you. I can hear you saying but I don't have a place to rent gear from in my city. Well I will make your life easy check out OutdoorsGeek.com and remember if you are one of my members you will get a code for 15% off anything you rent our buy from them.
There is also another good reason to rent gear, when it is something you will only use once, or even a few times, but not enough to justify the price.