These are the viewpoints of Shia scholars regarding your question:

Question: What is the ruling on the marriage of Muslims with Non-Muslims and Shias with Sunnis?

The office of Ayatullah Fazel Lankarani:

The marriage of a Muslim woman with a Non-Muslim man is batil (void), the marriage of a Shia woman with a Sunni man is makruh, the marriage of a Muslim man with a Non-Muslim woman is also void unless the marriage is a temporary one (mut’ah), and the marriage of a Shia man with a Sunni woman is okay.

The office of Ayatullah Bahjat:

Temporary marriage with the People of the Book (Ahlul-Kitab) is correct and as an obligatory precaution it isn’t permissible to perform the marriage contract of a Shia girl or woman and Sunni man.

The office of Ayatullah Sistani:

Getting married to the People of the Book isn’t permissible as an obligatory precaution. On the other hand, it is okay to get married with Sunnis if there isn’t any fear of going astray and losing Shia beliefs as a result.

The office of Ayatullah Makarem Shirazi:

It isn’t permissible for a Muslim to get married to a Non-Muslim, while it is okay for Shia men to get married to Sunni women, but taken into consideration that there are chances of going astray for Shia women getting married to Sunni men, such a marriage isn’t permissible.

Note: Marriage between Shias and some “Muslim” sects such as: The Ghulat, The Nasebis and The Khawarij, who falsely claim themselves Muslim, but in reality are Kafirs, isn’t permissible.[1][2]