... there's something that most people miss. Fitting. Just as you aren't comfortable wearing clothing too tight, or too baggy and loose, there's a proper fit for a bicycle, as well.
A bike too small will cause too much pressure on your butt and your legs never extend properly, causing stress on your knees.
A bike too big will be difficult to seat - getting to the saddle will be harder than it should be and your legs will hyper-extend during pedaling, causing pain in the knees and hips after having ridden for a while.
Because there is such variation in physique between people, and geometry between bikes, the bike that fits one person at 5'9" tall, may not fit another person of the same height.
The BEST thing you can do for yourself is to go to a reputable bike shop and let them fit you for a bike. Even if you don't purchase a bike there, you will have established, in your own mind, just what the right sized bike should feel like - you will know what to look for as you look further for your bike.
I believe that so many bicycles sit in garages and never or seldom get ridden because the bicycle is uncomfortable - so don't waste your money.
Know what to look for. Know what a properly fitting bike feels like.