Those two packages have different purposes.

python2.5 is basically a run-time utility. It gives you the python interpreter itself, plus the python "bindings" pyGTK (for Maemo's underlying GUI engine) and pyGame (for SDL, a popular graphics engine), plus a bunch of utility libraries and maemo-specific goodies.

Most users will install this, as you said, as a "dependency" : meaning, they just want to use some ready-made app (like iTablet or XGalaga) that was programmed in python, and it needs some parts of the above package to run. They may not actually know or care what python is, and that's just fine (think Visual Basic DLLs or .NET runtime in Windows).

IPython is only useful if you actually want to use Python, or learn it, and explore it as a programming language. It is an enhanced "shell", a command-line interface to the interpreter, but much improved over the standard one. It is a nice tool if you intend to play around with python code and/or use the tablet as a learning tool for the language, but in no way necessary to run python-dependent apps if that's all you want to do.
(edit: and of course it depends on the python2.5 runtime, it doesn't replace it, as the respective sizes suggest :-)