I think it'll also come down to your personality.
I agree with the posters that say it's really an individual thing whether a certain horse is better or worse as a confidence builder.
The thing I've found for myself is that I, as a person, really want to know "why" before I do anything. And the same goes for most mares I've met - most want to know WHY you want them to do something [however "dumb"] and once they know, they're perfectly willing partners.
Many geldings, on the other hand, seem to genuinely care less about the "why".
For me, since I appreciate the "why", I find I tend to get along best with mares who want to know "why" too.
I've met mares that could care less about "why" and I've met geldings where "why" is their favorite question.
I've also found that many mares seem to, I guess, "care" more. Maybe it's that maternal instinct coming out? I'm not sure, but I've found that mares can tend towards being more "emotional" than geldings. For instance, I've never fallen off my mare because she HATES people falling off. As a result, she consciously works to keep me on when I'm in danger of falling. There have been many times when I should have fallen and didn't, solely because she didn't allow it. For me, that's a huge bonus since I love feeling like my mount is "awake"...but some people really hate that!
Of course, some geldings really care too. It's not a sure thing that a mare will care and it's not a sure thing that a gelding won't!
Anyway, for me, my confidence builder is a 29 years young Arabian mare who is prettttty darn hot on the ground, but would never dream of hurting anyone or being anything other than perfect [but quietly opinionated] undersaddle.
My personal fear is cantering and she's never once missed a step while cantering. She's never bucked, never slipped, or done anything else to make me feel less than safe while we canter.
I used to think my ideal steady-eddy was an older Quarter Horse gelding who was mostly interested in grazing and going with the flow. However, I've changed that and now my ideal steady-eddy is a been-there-done-that, energetic, old-lady Arabian mare!
Anyway, don't discount a certain breed or gender because of a stereotype. They might be JUST what you need. :)