The Bottom Line: Each Horse is an Individual
Just like people, we all know that horses have their own personalities and individual quirks. It is unfair to generalize that all geldings are one way and that all mares are another way.
I think most people's preference for mare or gelding depends on their personal experiences with horses. If their best horse was a gelding and worst ever was a mare, you might be a little tainted when it comes to mares.
Over the years, I have met wonderful mares and wonderful geldings. I honestly can't say that I have a preference. I know some people have a strong preference, especially if they are looking for a horse to buy or even lease. Not me. I look to see if they meet all my requirements, and if they do, it doesn't make a bit of difference to me whether they are mare or gelding.
Mares: What You Need to Know
Mares, in general, are known for being more sensitive and more temperamental than geldings. When they come in heat they can be extra sensitive and persnickety. I guess you could say they experience the sort of thing us women do during our "time of the month".
Some experience this in more dramatic ways than others, ways that might affect their performance under saddle. There are medications that veterinarians prescribe that can help mares with this. It can get expensive to maintain them on these medications though. And most mares get along just fine without any sort of medication to help them manage their hormones.
I have found mares sometimes need to be asked instead of told. They are more sensitive to our energy and tend to get tense and feed off of our feelings a bit more than geldings do.
In my experience, mares tend to get more attached to other horses than geldings do. I have seen more herd bound mares screaming and worrying about their other horsey friends than I have geldings. Not that there aren't herd bound geldings, but in my opinion, I see it more in mares.
I have found mares to be more perceptive than geldings. Though they can be a handful and a real pain when they want to be, I have found that when they are "on," they are "on". A mare having a bad day may be your worst nightmare, but when she is giving you her all, they will give you 150% of themselves.
I used to say about my heart horse, Zelda, that she was either terribly awful or amazing. Everything was very black or white – we never had just okay rides. It was either we wanted to kill each other or we felt like rockstars we were jiving so well.
In my opinion, mares tend to get more attached to or "bond" more with their owners or riders. My mares have been the only horses I have had that ever showed an interest or acknowledged that they "knew me."
Obviously, all of my horses know me at feeding time, but I'm not talking about that sort of acknowledgment. I mean things like walking up to me or coming to me when I call them, or even nickering at me, or following me around without being led. Again, this is just my experiences with my mares.
Geldings on the Other Hand...
Geldings on the other hand appeal to a lot of people because they are generally more even keel. Whatever their personality is, or normal energy level is, they stay pretty consistent. With a gelding, you can pretty much count on the horse you pull out of the stall or field will be consistently the same on any given day.
They don't have the disadvantage of the hormonal changes that the mares have that cause them to have mood swings and drastic changes in behavior throughout the year.
Geldings I have found to be more forgiving to a rider's mistakes. Not that it is right to be overly aggressive with your training methods, but I have seen geldings take a lot more "abuse" or rough handling than mares.
Though they are perceptive to our energy just like mares, geldings are less reactive to it than mares are. I know I have been in a bad mood or gotten frustrated with my geldings and ridden them really hard, and they just took it, unlike my mares who would have not tolerated me taking out my frustration on them.
I have not seen as many herd-bound geldings as I have mares. They will have their friends out in the field of course, but I have found geldings tend to be more independent and less worried about other horses than mares usually are.
I have also found that my geldings don't seem to care as much about me or bond with me. Not that they don't know who I'm or recognize me, I can just tell that they aren't as interested in interacting with me as my mares have been.
The gelding is more workmanlike – they like to come out, do their jobs, and be left alone. Whereas the mares have tended to enjoy the attention more.
So What Should You Choose?
When looking for a horse, you should decide the skills or experience that you want the horse to have and then make a list of non-negotiables.
In my opinion, looking exclusively at only mares or only geldings is going to have you missing out on a lot of nice horses, ones that might be a good match for you.
My advice is to be open-minded when it comes to whether you want a mare or a gelding. Remember, just like people, or dogs, or any other species of animal, they all have their own unique personalities and qualities. When you find the horse whose energy you respond to the best, you will know. So will your trainer. Don't let a small detail about whether the horse is a mare or gelding keep you from a horse you have a great connection with.