Is PSOLA or WSOLA better for vocal stretching/retuning?
To be more precise, I'm trying to work out the advantages and
disadvantages of WSOLA vs PSOLA for the following tasks:

Pitch correction:

PSOLA - Advantages you can change the pitch directly based in the rate space between  grains, keep formants intacts...  to change the pitch you not need time scale and  resample after.
PSOLA - Disadvantage do you need a super power pitch detector
WSOLA - Disadvantage WSOLA is not one Pitch Shift algorithm, WSOLA is one Time Scale algorithm, to change the pitch do will need time scale and them resample, the formants will be lost, so you just maybe need one kind of autocorrelation to find nice places to slice, so it is not the same as Pitch information, if are you building an Automatic Pitch Correction one basic autocorrelation will not give to you the correct Pitch Information.
WSOLA - one advantages here is that it is not Pitch dependent to slice, just slice in the best match similarity, remember it is just one time scale algorithm, you will need resample to change the pitch

Time correction:
For time scale I think the WSOLA have best advantage here or some TDHS based algortihm, yeahh you can use PSOLA to time scale and apply pitch shift together, just pitch shift or just time scale , but if are you just time scaling, WSOLA is one smart choice, no need build an Ultra Pitch Detector in WSOLA, just one wave form similariry will solve your problem (AutoCorrelation can solve ...)
Formant correction:

PSOLA - by default formants will be untouchable, but you can warp the formants too, it is amazing in PSOLA ...
WSOLA - after Time scale and resample your formants will be lost and to get the original formants you will need get the formants in the spectral envelope from original signal and apply it in the Pitch Shifted signal, you can do it using an low pass filter in the spectral, use a peak picking and interpolate or use a cepstrum to get the envelope ...

I know that PSOLA is infinitely more complex, but I consider this algorithm really great, if you have a decent PitchTrack it will give you great versatility