Online cube solvers and mobile solver apps solve the same basic problem: you enter a cube state and get a move sequence. The difference is where and how you use them.
If you are at a desk, an online solver can be fine. If you are holding a cube on the couch, at school, or on a train, a mobile app is usually easier.
Online solver strengths
Online solvers are easy to open from any browser. They are useful when you are already on a computer, researching cube notation, or comparing different solving methods.
They can also be convenient for copying and pasting facelet strings if you already know the technical format.
Mobile app strengths
A mobile app is better when your real cube is in your hand. You can enter colors by tapping, follow the move animation on the same screen, and reopen the app quickly next time.
Rubix Solver is mobile-first. The flow is designed around touch input, one-move-at-a-time playback, and 3x3 or 2x2 selection.
The deciding factor
Choose the tool that reduces mistakes. If a website makes you lose your place, use an app. If an app does not support your cube size, use another tool.
For most casual solvers, mobile is the friendlier experience because it stays close to the physical puzzle.
