There is no difference between the Reading Eggs web version and the app in terms of the content in the program. As an online program, the app simply provides an alternative way to log in.
In terms of registration and purchasing there are a few small differences.
If you register for a trial via the App Store or Google Play, you automatically enter an ongoing monthly subscription after your free trial. This will be deducted monthly through your App Store or Google Play account until you manually cancel it. If you register via the website, you do not enter an ongoing subscription agreement and your access will simply discontinue once the trial period ends.
If you purchase a subscription via the App Store or Google Play, you will only receive the option to enter an ongoing monthly subscription, which gives you access to both Reading Eggs and Mathseeds. If you purchase through the website by logging into www.readingeggs.com you will be able to choose the same monthly subscription option plus some 12-month subscription options.
No matter which subscription option you have, you will be able to access the program from either the website or the app.