Haven't seen any difference at all in fundraising abilities of a PTO versus a PTA.
If you're planning on soliciting straight donations, there are some advantages to being a 501c3, but many PTOs get that status. Same is true of some grant opportunities that are only open to 501c3s.
As JHB also pointed out, many parent groups utilize the school's own non-profit status (operating in effect basically as a committee of or a part of the school) rather than becoing entirely independent. The Lowe's Toolbox for Education grant program, for example, allows independent 501c3s to apply for grants and also allows grant apps on behalf of the school itself.
I haven't seen PTO/PTA make any difference at all and I've been involved in both.
Frankly, most people don't really understand the difference anyway. But that's in regard to sales and event type fundraising where you are selling items or holding an activity.
As far as getting donations/grants, we have seen a difference if the organization is not a 501(c)(3) charitable organization. Many companies won't even discuss donating or supporting the parent group if you can't supply that proof. (But some still will.)
Depending on your organization, you may be able to circumvent that if your school is the partner and the donations are for the school. If it's something like food for teacher appreciation or items for silent auction, it doesn't much matter who's name the donation is in (school/parent group) if the two are partners on the event. But for money - if it's given to the school, then the parent group can't really use it.
Our high school / district wouldn't allow us to ask for any donations using their name, but we had much better luck after we became a 501(c)(3).
