I have not had a mare spayed so I can't help with pros/cons.  I have a friend who did take her mare to Vermont to have it done and was quite pleased - the mare was positively obnoxious during her heat cycles.

Is your mare's attitude a problem only when she is in heat?  If so, here are a couple of non-surgical things you could try (if you haven't already)  Talk to your vet to see if any of these might be a viable alternative.  1) Insert a sterilized marble to "trick" the uterus into believing there is a pregnancy.  This is quick, inexpensive and lasts about 3 months at which point the uterus will kick out the marble and you simply do it again.  2) Depo Provera (marketed as birth control for women) given every 2 weeks is effective in suppressing heat cycles although I do not know the long term ramifications on the mare's future reproductive soundness.   3) Altrenogist (Regumate) given orally daily.  Very effective but the Regumate MUST be handled with gloves so that it is not absorbed through your skin.  Both the Depo and the Regumate can be a bit pricey.  (I'm a big fan of the marble!)  Talk to your vet to see if he/she is comfortable with any of these methods and if they may work for you.

Good luck to you!

I had a two year old mare spayed  several years back because her breed association would not issue regular papers because of a excessive white issue.There are a few ways to do this, through the flank or belly, we chose the belly as no scar would show, also chose removal of overies not just tying to prevent any hormone issues. The rusults were great, the mare acted as a gelding, no hormone problems, no weight gains as was stated from previous post in short no problems at all. This mare went on to show and led the Nation in her breed association in SB Pleasure and continued showing until her retirtement. Most people never knew she had been spayed.