How would I know which assault charge to file?

Nonsense. That is a sexual assault. go to the police, and file charges. The cops know what to do. As they say, 15 will get you 20.

Sounds like your question is for the criminal forum. But if you intend to sue for civil damages, here's the applicable statute: 1708.5. (a) A person commits a sexual battery who does any of the following: (1) Acts with the intent to cause a harmful or offensive contact with an intimate part of another, and a sexually offensive contact with that person directly or indirectly results. (2) Acts with the intent to cause a harmful or offensive contact with another by use of his or her intimate part, and a sexually offensive contact with that person directly or indirectly results. (3) Acts to cause an imminent apprehension of the conduct described in paragraph (1) or (2), and a sexually offensive contact with that person directly or indirectly results. (b) A person who commits a sexual battery upon another is liable to that person for damages, including, but not limited to, general damages, special damages, and punitive damages. (c) The court in an action pursuant to this section may award equitable relief, including, but not limited to, an injunction, costs, and any other relief the court deems proper. (d) For the purposes of this section intimate part means the sexual organ, anus, groin, or buttocks of any person, or the breast of a female. (e) The rights and remedies provided in this section are in addition to any other rights and remedies provided by law. (f) For purposes of this section offensive contact means contact that offends a reasonable sense of personal dignity. (Added by Stats. 1990, Ch. 1531, Sec. 1.)

The prosecutor determines what degree of assault to charge. However, it is likely he would be charged at least with Assault 4.

Normally you talk to the police the DA and the magistrate about such things. Why is it you think you have to worry about such things?

Go to the police and tell them what happened. The police will determine the type of assault.

If you are referencing criminal charges, you do not decide. You or someone who actually has direct knowledge of the facts reports the crime, in this case an assault, and law enforcement and the assigned prosecutor will decide what charge or charges they believe the available evidence may warrant. If you are referencing bringing a civil action, then you are referencing bringing a civil suit on the intentional tort, i.e., the inappropriate touching.

Make the complaint to the police or prosecutor, the prosecutor determines the charge, not you.