Why not both? Assuming it's running off a DB in the cloud, it should be simple enough for the developer to write a web front-end for it.
I try to keep my phone lean and mean, and I'm not inclined to install an app for something I don't use all the time. Heck, even if I were to use this all the time, I would much rather use it through a web interface.
The answer probably always differs, but for me I'm just one developer without any mobile dev experience and plenty of site features to keep working on -- so taking time away from the main product to work on a complex app equivalent (and implicitly commit to update/maintain it over time with each new feature) didn't make a lot of sense.
I try to keep my phone lean and mean, and I'm not inclined to install an app for something I don't use all the time. Heck, even if I were to use this all the time, I would much rather use it through a web interface.