If I may, some more info. As WBB89 said,
With that said, a 70-200 f/2.8 or 300 f/2.8 are very rarely a bad choice for sports.
Especially outdoor sports.
Con - The 300 2.8 is a bear to carry, it comes in at 102 oz (6.375 lbs) [2.98kg]. A Monopod is practically a necessity, hand holding gets tiring FAST.
Pro/Con - The 100-400 Mk I is a push-pull zoom, while the Mk II 100-400 is a twist zoom. It's important to some people.
Pro - The IQ difference (IMHO) is negligible on 70-200 and 300 (I own both and 1.4 II). I have not shot a 100-400 nor used a extender.
On to the sport...
I've shot many seasons for a CARFU Senior Mens Div III team. Thats the lowest division in organized rugby in the USA, a small local team. I shot many of those seasons with the 70-200. What I learned was to cut down the field. I would roam the touchline between the try line and the 22 meter line. I would draw an imaginary line down the center of the field. Anything on the far side was not to be photographed. I would wait until they were on "my" side of the field. Additionally, I would limit the range I shot when they were on "my" side of the field. If they were on "my" side, but 60 or 70 meters away, I would refrain from pics. Did I miss some good shots? Of course. All photographers have stories of the "one that got away".
As I said this is a small team so I had complete access to the field and could go where I wanted when I wanted. Your mileage may vary depending on field access.
Another argument AGAINST shooting across the field. 200mm f/2.8 @ 50 meters gives a total DOF of 6.65meters (21.7 ft). Depending on your sidelines this can make unfortunate sideline clutter. The 300mm f/4 @ 50 meters gives a total DOF of 4.2 meters (13.7 ft). SOURCE
So to gives my two cents....plus a lot more. sreer said it best.
I would choose the EF 70-200mm f/2.8L IS II USM and get the 1.4x III extender. You get some length with the extender and 2.8 for indoor sports on a fast focussing lens. I would definitely rent and try first.
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