While tonality is preference based: The dynamic range, sense of space, and 'slam' created by a headphone can't be mimicked in an IEM.

In general: Speakers > Headphones > IEM at similar price points.

However, it depends on the listening environment. Most good headphones are Open Back. My house is old and my home office overlooks a busy street. I can't enjoy Open Back headphones due to the noise pollution. And I can't focus with speakers.

So I have a semi-open headphone that works most of the time (E-mu teak), an IEM (B2Dusk) for when its busy or I want isolation, and a pair of noise cancelers for piping in white noise when I have to focus on a high functioning task.

I have also put the nose cancellers overtop the B2Dusks for more or less complete silence.

TL:DR - Depends on use case more then anything else.