Which lens shall I go for?

The non IS is an old design (dates back to 1995) but is still considered to be an excellent performer and for your purposes I personally would go for that one, shooting equestrian you will probably want a higher shutter speed so IS wouldn't be needed, add a monopod for the occasions that you will be shooting a lower shutter speed.
Of course should you be shooting at slower shutter speeds most of the time then the IS version would be useful....
Also it depends on the price of each, they will both perform well.
Of course should you be shooting at slower shutter speeds most of the time then the IS version would be useful....
Also it depends on the price of each, they will both perform well.

More info needed. What body will you be using and might you upgrade in the near future?
The IS is undoubtedly useful, but its likely to be of great benefit when taking shots of moving subjects such as horses. That said, the L series lenses are of usually excellent build quality and an older version would trouble me greatly. The one caveat is the spec of the body used with it. Its possible that the newer high resolution bodies may out resolute the older lens (this could be an issue with the non IS model too so its worth checking).
Edit: one additional thought - the new lens will likely have a warranty.
The IS is undoubtedly useful, but its likely to be of great benefit when taking shots of moving subjects such as horses. That said, the L series lenses are of usually excellent build quality and an older version would trouble me greatly. The one caveat is the spec of the body used with it. Its possible that the newer high resolution bodies may out resolute the older lens (this could be an issue with the non IS model too so its worth checking).
Edit: one additional thought - the new lens will likely have a warranty.

The Mark 2 IS version was a significant improvement on both the mark 1 and non IS, additionally, I am not too sure how long Canon will support those older versions, support is normally withdrawn 10 years after production ceases
Don’t know how much mark 2 versions are now, but the mark 3 was introduced a while back and although it only had cosmetic improvements, it may mean that the mark 2 versions are now slightly cheaper
I believe both sigma and Tamron have offerings at f 2.8 as well and that they surpass the older Canon versions, but not the mark 2 or 3, so they may be worth checking out also
Don’t know how much mark 2 versions are now, but the mark 3 was introduced a while back and although it only had cosmetic improvements, it may mean that the mark 2 versions are now slightly cheaper
I believe both sigma and Tamron have offerings at f 2.8 as well and that they surpass the older Canon versions, but not the mark 2 or 3, so they may be worth checking out also

I recommend you check out Tamron's G2 70-200 2.8 VC lens. It was equal to the Canon in center sharpness and better on the edges
in a detailed review by Lens Rental. I own this lens and love it. It is also excellent with Tamron matched 1.4x converter (see my just posted image of barn swallows). Also about $600 less than Canon.
in a detailed review by Lens Rental. I own this lens and love it. It is also excellent with Tamron matched 1.4x converter (see my just posted image of barn swallows). Also about $600 less than Canon.