My personnal impression is that the Pirc has a better reputation at clublevel (it is plaid more often = more players believe in it than in the Alekhine).
I do not know at which level your are playing but there are two two you certainly have to know:
- both the Pirc and the Alekhine are quite difficult to play and you will need some experience before understanding them correctly
- you have to feel comfortable in the arising positions to be able to handle them. The Alekhine nor the Pirc is everyone's cup of tea
It is also easier for black in some variations of the Pirc to play very aggressively just from the beginning of the game. With the Alekhine, you will have to be more patient in (nearly) all variations.
I have used for years the Pirc as secund defense and it is quite usefull (for example when white "refuses" the Alekhine with 2.Nc3) and so I try to get the same type of positions in the Alekhine by playing g6 when it is possible (modern with 4. Nf3 and exchange variation with 5.ed6-cd6).
A problem of both defenses is what to do when white "declines" them. When playing the Pirc, you should have some knowledge of the closed sicilian and related systems. With the Alekhine, you will have to know more: the Pirc and closed sicilian (in case of 2.Nc3), the French (2.d3) and eventually the Vienna or the e4-e5 games when countering 2.Nc3 with e5. So I'd say it is a bit more difficult to be good prepared when playing the Alekhine.
Another thing is that both these openings are quite addictive. Once you strat playing them it is quite difficult to stop with them even after having being crushed.