The best answer would come from someone who has worked for both companies. I have not worked for either.
A majority of Amazon jobs are in Seattle, while Uber is in San Francisco (soon to be moving to Oakland).
I would look at sites like Glassdoor to see what people say about each company. Plus if your interview at each of them you could get a sense of how it would to work for them.
Find someone on LinkedIn who works at either Uber and Smazon and see if you can take them to coffee and ask their opinion.
Source: Quora