I understand that the latter is a slightly better lens, but it's almost twice as expensive. Is it worth it for the additional 50mm, USM and build quality?
The focus is faster, right. Only you can answer if it's worth the extra cost. The build quality is not weatherproof so I would not call the build quality much higher.
The extra 50mm may not matter, as the image quality of full-frame lenses is typically slightly worse when used on crop bodies than with full-frame lenses. Therefore, an image taken with the 55-250 at 250mm may be almost as good as an image taken with the 300-300mm 300-300, even when extra trimmed.
If you plan to use the lens only during this safari and live in an area where renting a lens is an option, you should rent some of the better lenses like Canon 100-400 or Canon 400 prime. I do not know how big the planned animals are and how far away they are. I would choose the Canon 400 prime for small animals like birds and Canon 100-400 for big animals.
Well, of course, if you're planning on taking photos of wildlife more often and are not ready to buy an expensive lens, the 55-250 and 70-300 lenses may be a good choice. I would also add a third-party 100-400 lens like the Tamron 100-400 lens. Third-party lenses sometimes have compatibility issues (example: My Tamron 100-400 had problems with Canon EOS RP), but it often fixes newer firmware (example: My Tamron 100-400 with the latest firmware works fine with Canon EOS RP). ,
My experience with shooting 55-250mm birds is that I used it all the time at 250mm. At least for birds, a lens with 100-400 could be a better choice.
A good middle ground could be to hire a lens that you can afford if you like it. For example, rental of lenses offers Tamron 100-400. The Tamron 100-400 is weatherproof, unlike the Canon 55-250 or Canon 70-300. In addition, the Tamron 100-400 has a focus limiter, unlike the Canon 55-250 or Canon 70-300.