Should i "invest" in proofs uncirculated or circulated coins?

Started to buy coins for there silver value and almost that entirely. If i run across a nice coin at about the same price i will but it and hope the condition will boost value ,but im mainly in it for the silver content.I have only bought silver quarters and silver kennedy half dollars off ebay and mainly if i can get the coins for basically spot value with shipping included and ive done well this way.Also ive bought a handful of american silver eagles.My question is should i just buy loose kennedy's an silver quarters that are not in protective covers since im mainly looking for silver or should i still be looking for coins in protective cover?Thanks