RE the long lenses
Go for the zoom. The Canon 500mm is a much better lens, but it's also more of a pain to transport, more of a pain to carry, insurance is far higher on it (needless to say if anything bad were to happen to a lens while in your care, it's much better if the 150-600 is damaged than a 500mm), but most importantly, it takes much longer to get used to.
Since you say you don't have experience with that kind of lens, the zoom will be much better for you. I can not stress enough that, for your first time using it, you will completely underestimate how tough framing is with the 500mm f/4. Everybody also underestimates how tough it is to not just frame a shot but keep that framing while you rattle off a few frames; capturing moving wildlife with the big white telephotos is one of the hardest techniques to learn. Wildlife photography with telephoto lenses may not have much 'art' to it, but it's one of the most technique-demanding areas of photography. You will not nail it on your first try, so you may as well make it as easy as possible for yourself and ease yourself in. A zoom is perfect for that.
For reference, when filming from long distances with the BBC, zooms are used over primes, specifically because trying to get the framing right with a fixed 500mm or 600mm is just too hard to rely on, even among experienced professionals. Obviously you'll get individuals who like to use those big primes (I do, myself) but in terms of getting the job done reliably with no mistakes, time after time, zooms are the standard.
Yes, the 500mm f/4 is optically superior to the zoom. But the #1 influence in image quality when it comes to wildlife with long lenses is getting as close as possible. The Tamron 150-600 is easier to move with, easier to frame and shoot with, and thus easier to get close with. This is why lenses like the 200-400 f/4 and 400mm f/4 DO are so much more popular than the 600mm f/4. It's not all about reach and optical perfection; practicality is king. Optics mean nothing if you're so far away that heat haze blurs everything, or if the lens is too awkward for you to frame the shot.
If you do rent the 500mm, you'll regret not having the easier zoom. If you rent the zoom you'll likely keep thinking to yourself "man, I wish I had that big fancy L lens". Neither is objectively all-round better than the other. But for a new user, the zoom is much, much easier. Rent the zoom this time, and if you find out you like shooting with long lenses, buy the cheap 400mm f/5.6 to practice with and then rent the 500mm on your next big trip.