Definitely keep the private keys but I also think there is value in keeping the .dat file.
In many cases there may not be an up to date snapshot available and you having the file might be useful for the revival of the chain.
Paying for a drive sucks but it depends how "conservative" you want to be about the risk.
This is a question we dealt with in our discussions for our project.
Disclaimer: I am running an ICO now that's sole purpose is to deal with this issue of deadcoins.
Have a look at what we're trying to do and any feedback is much appreciated!
https://www.coinjanitor.io/