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 - gone but not forgotten
2015 cayman GTS - new toy
2006 SLK 55 AMG sold