Personally I opted for the 500/4 IS instead of the 600/4 IS, mostly due to the difference in size and weight.
The 500/4 is usable on a standard, heavy duty ballhead in combination with a gimbal accessory such as the Wimberley Sidekick. The larger 400/2.8 and 600/4 (Mark I's) are not recommended with that, really should be used with a full size gimbal head. That head is more expensive and it's not as quick and easy to swap the head on the tripod, so the tripod is more dedicated to long-lens-only use.
I use Gitzo 1325 and 1348 tripods with my big teles (among other things). Those are now discontinued models that are still excellent and fully usable, can sometimes be found used at a considerably more reasonable cost than the current models.
Aside from the size and weight, my biggest concern with oine of these older lenses is repairability. Canon will only support a discontinued lenses for a limited period of time, so if something breaks it might not be possible to get the parts to repair it. There is no set period of time... it can be many years, or just a few. This occured with the 200/1.8L. Shortly after it was discontinued parts supplies for it's AF mechanism were exhausted, so if a lens' AF happened to fail, it became a big, expensive paperweight. That's a rather extreme example and other lenses have had a longer repair history after being discontinued, thanks to adequate stock of spare parts.... but it's always a bit of a risk.