I guess another factor is if you use a case or not. I don't use a case but I do use a screen protector. With a case you have more protection for the screen. But since I don't use a case it helps having the protector since the screen is always touching a surface. I don't have to worry about being careful with my phone, and I've always dropped the phone before. I wouldn't say I'm clumsy but I do have a hard time holding onto a caseless iphone 4 sometimes.
Currently, I'm using the power support crystal clear protector, and have since the first iphone. They do not degrade the screen and the feel of it is something I prefer. Its not like some other protectors which can feel like an orange peel.
Also, the benefit of being able to remove the protectors and sell the iphone as perfect is awesome. I've spent very little on iphones but have had every single one. If you keep the screens perfect, unlock and jailbreak them, you can get a huge return on your initial investment.
But when it comes down to it, it's all about what feels good for you.