SSPVer here. I can not speak for MHTS but figure that many others can. If you go to the Immaculate Heart Seminary, you will have to become a member of the Congregation of St. Pius V. That's the most important difference. CSPV priests live communally and travel to the missions by means of car or airplane. MHTS only lasts seven years whereas IHS lasts eight years. In terms of courseload, they should be mostly the same. IHS' staff is mostly, if not exclusively, "new guard" priests, whereas I am to understand that MHTS has a few "old guard" priests left.