I'd say the main reason to use Gloo is it's much simpler, it's based on Envoy which is where most of the innovation around L7 proxies is happening these days (ie, see WebAssembly), and was built from the ground up with a cloud-native and kube-native architecture.

More can be found here:

https://docs.solo.io/gloo/latest/introduction/others/

And I can go into much more detail on slack. I'm `ceposta` on the solo/istio/cncf/envoy/kubernetes slack