Hi Pauhana, I also primarily use the 7dii and made over 60,000 shutter activations with the old Canon 100-400i before putting about 15,000 on a Sigma 150-600 Contemporary. I will say that I am about ready to go back to Canon though. The Sigma is a very good lens, but I have found that it tends towards having the same problem of not working exceptionally well in lower lighting situations as the Canon 100-400i did with the 1.4 extender. The main reason for purchasing the Sigma was that I got it used in excellent condition for only $500 and traded in the Canon for a net profit. The cost of a new mkii lens is the one factor still holding me back.
I don't have anymore experience with the Canon 100-400ii other than one quick outing when I borrowed my brother's lens, I did decide that the Canon 100-400 in any model, is preferable to the Sigma 150-600 in all ways even without the extender except for the purchase price. I found that it was much better at locking auto focus on and holding lock on birds in flight. Also, the Sigma is heavy. Especially if your going to be hand held, it will be a factor. Another factor that you may not have considered is that North Dakota will have a lot of wind to contend with and the Canon is smaller and hence pushed around a lot less than the Sigma. Also, the Canon will give you more reliable focus and sharper images at longer ranges and hence I have found that even at 400mm you can crop to an equivalent and often better image. I didn't try an extender when using the Canon mkii, but my brother said his experience was that it worked fine. This is often the better approach anyway as at 600mm your field of view becomes so narrow that it is often hard to find that bird in flight.
In summary, I would recommend going with Canon L glass, especially since you are renting. Also, you will get a lot of bright sun and vast flat open prairie, so take along the 1.4 extender as in that lighting, it should work for you. What routing are you planning, if you don't mind saying? Maybe I, or someone else, can make some suggestions for some quick birding along the way.