Depends. If I'm buying something to sell on then definitely a gelding as so much easier to sell. If I'm buying something for myself to compete then either - often mares are tidier jumpers and try really hard for you! But then geldings are more straightforward...
I love stallions! They used to have a REALLY posh Lipi stally at the yard, and he was the sweetest thing ever! But it's not really practical for us to go for a stally xD
I used to only have mares and now have two geldings, so went for the third option, but 2 years ago it would have been mares only - something about them, but I can understand why some people won't touch them.
I have a problem (well 2 actually), see I have Touchy (orange one below) and Sovereign (dark bay one below) and both are perect but T is a mare and S is a gelding! So what would you decide!
My mare *is* mareish. In fact her 'nickname' is Queen of the Harpies.
She is bossy, pushy, stubborn, argumentative and difficult. If she doesn't like something she will let you know!
But she is also hillarious, very loving and has a huge complex personality that never gets boring. I love her, I understand her (she is just like me in a lot of ways) and we suit it each other down to the ground. I am total mare convert and I don't even think I've got a nice one LOL