There are a few big differences. IPL will hold all of your photos and videos indefinitely. PS only holds photos and only holds them for 30 days. It also has a max of 1000 photos. Photos and videos stored on IPL take up space in iCloud. Photos in the PS do not take up iCloud space. IPL also can help you store and access more photos and videos than your device can hold because it can be configured to only keep a low res copy of the image on your device to save space.
Personally I think IPL is a much better solution and I'm looking forward to using it, but I think PS could still be useful in some situations and I think the two features can coexist.
In the previous betas, though, if you turned IPL on it would automatically turn PS off. I'm not sure there's a need to have both on at once. I'm not sure if the fact that they're both enabled at once is a bug or just a change in beta 5.