Above ground is the safest.
Above ground storage has the benefits of:
1) monitoring the containers for leaks
2) dissipating the heat from the waste
3) "Spills'" can be contained and cleaned up. (There's not much chance of chasing a spill down through the salt or rock to clean it up.)
4) cheaper
And storing nuclear waste above ground would bring the problem to light, instead of hiding it.
The major obstacle is "NIMBY"s.