Hi Cameron, PTA voice here... since your group is so small, (you did say 100 total students?)you could definately benefit financially as a PTA unit. You do get a significant discount on insurance for your group and it is likely that you would spend more money on insurance as a PTO, then you would as a PTA paying dues + insurance. ( I believe the insurance rates are low in California through PTA most likely no more than $200 and since CA units are many that amount is probably high.)
In addition, you are also in a state where your PTA is extremely active. Trainings, financial assistance, automatic 501c3 status under the PTA umbrella. You wouldn't have to pay for a 501c3 to start up and bylaws would be automatic and would only need a few adjustments from your group. You will also likely only have to contact the California PTA and they will assist you in setting up.
I disagree with Tim in that you WILL have a hard time should you choose PTA and then change your mind. I know that he has had contact with groups that have a had a hard time but I also know that there are groups that have not. It is likely he would hear from the unhappy ones most often as he is the founder of PTO today and who would want to tell him and why would they, that the PTA was actually pretty ok. If you would like more information regarding what PTA is and does, for both National and California(There is a lot of local focus out of the CA PTA),
www.pta.org
and
www.capta.org
You can also call them and ask that they send you some information at (213) 620-1100. There is also a webform at the CA PTA site but calling is quicker.
Contrary to KJ's belief, I think PTA is perfect for small schools like yours because you actually save quite a bit of money being a PTA, the group still has the same local goals, you just get better perks because of your numbers.
I hope that I have given you enough information for the PTA perspective but I do hope that you do research both and choose the one that you think best fits your group. Either way, any parent involvement is good!