It’s a very good question, and one that gets asked about once a week in these forums, so you can find lots of existing threads on it. 
I don’t know how difficult it is for you to get the devices to begin with. If returning them would be easy, my recommendation is the same as I have posted elsewhere: just get one of each and try it. There are some differences and different ones will work better for different people.
If returning them would be too difficult, then I would say if you are already using Chromecast and you are already using Google assistant on your phone, then get the Google device. If not, my personal recommendation would be to get the Amazon echo device.
Both are good devices, both are continually adding new features, so really both are good choices.
For more details, see the existing threads, there is a great deal there already.