What are the differences of a ferrite core transformer and an iron core transformer at low frequencies?
In general a transformer with an Iron core has a higher saturation flux density so you can get away with a smaller core, or fewer turns in a low frequency design.  Iron cores do suffer from circulating currents in the core however which makes them lossy at high frequencies.  Iron cores are often laminated to reduce this effect.

Ferrite cores will saturate at a lower flux density but has lower losses at high frequencies as they have much higher resistance.