I agree, do both.
The M school is a nice introduction at low risk to you, and if you like it, the HPDE's are cheaper per event ( $ 600 / day for me) and you can do many different tracks. The instructors are usually good, especially in BMW CCA or PCA events.
2011 e92 M3 Monte Carlo Blue ESS 535
2015 cayman GTS
2006 SLK 55 AMG