I'd say it depends on your travel. If you're always in or near a car, you can take a heavier lens. If you're going to hike, you'll want something lighter.

I'm going to assume you'll still take your 150-600 with you, as the large expensive lenses tend to primes, which can limit your framing and opportunities. Your experience with the 150-600 on previous trips should point out which prime focal length works best, so you'll get the same framing, but perhaps sharper, and thinner DOF. Maybe you could rent one of the newer DO lenses, as they're much more portable.

Another option is to perhaps rent the 200-400mm + 1.4x. This would let you leave the 150-600 at home, so you only have one heavy lens. This would give much the same focal length as you're used to... but with Canon's focus, top tier sharpness (I would assume...). The downside is it's only a moderate aperture advantage compared to your existing lens vs. something like the previously suggested 300mm f/2.8