my experience with screen protectors is that you want the soft rubbery ones which have a little bit of stretch to them. you do not watn the hard plastic versions which often are very cheap. the soft ones adhere better but are a bit harder to put on (they typically include a squeegee). i've had. the type i have on my smartphone is labled in bestbuy as "invisible shield" and certainly is not cheap but it has lasted over 2 years without coming off. the alternatives? even though they were cheap they just do not last.
as far as cases go you want silicone or a hard rubber. you do not want plastic. you will need to take the case off every once in awhile and silicone will prevent any real damage to the case. it also attracts dirt like a magnet. hard rubber is what i use and after a few years some parts will start to crack but it holds up well and protects the phone well. plastic will crack and fall off much easier. it also provides no impact reduction in case you accidentally drop your phone.
i wouldnt care so much about price but fit (a poor fit is a poor investment) and what materials they are made from.