I have an 18 yr old mare that has had 10 foals (all bred before I owned her) and I can't even tell when she's in season. Honestly. I think she's happy not to be pregnant for once!
The only time I could tell she was in season was when she shared a trailer with a gelding she just met. She thought he was cute. Other than that, I swear I can't even tell when she's in season, much less worry about her being moody or irritable. She is awesome! I really lucked out. She was pregnant when I bought her (which I didn't know) so for all I knew she could have been a real hussy. I was just blessed with a horse that was meant for me.
Geldings are great too. One of the ones I have right now is quite simply the best horse I have ever owned.
Now I have seen mares that were real hussies, squealing and kicking at other horses and acting like they are in season constantly. I would sure rather not have one like that. I have a name for them, but is probably not appropriate for this forum. Let's just say they are real dogs!
But a good mare is as good as any gelding. Some might say better. I can't really say because I've owned more geldings than mares so I am rather partial to geldings. But I wouldn't hesitate to own another mare if I thought she was a good one!