Adobe's Lightroom may be marketed as a professional photo editing app, but it's also the best photo library software out there, even for those who love Apple's Photos app.
We’re specifically talking about Lightroom Mobile for your phone or iPad, and Lightroom CC for your Mac or Windows computer. Like Apple’s built-in Photos app, Lightroom syncs between all your devices, keeping original photos in the cloud. But it also adds tons of more editing options, saved filters, and a great camera app that can shoot raw images.
Lightroom is the only professional editing/cataloging app that syncs between mobile and desktop (ON1 has a mobile version too, but it's still lacking in terms of camera support and features). You can import your images to an iPad while you're on the go and edit them there, and have everything come back to you on your Mac or PC.
Or you can do it the other way around, importing photos on your computer and then previewing, selecting, and editing them on your iPad. The default setting uses something called Smart Previews on mobile, which takes up much less space than the original raw files but still gives you full editing control.