In Digambar Jain tradition:

• Ṣaṭkhaṇḍāgama (Shatkhandagam) is older.

- Composed by Āchārya Puṣpadanta and Āchārya Bhutabali, generally dated 1st–2nd century CE in our era. ( jaingpt.org)

• Mūlācāra (Mulachar) is a later text.

- Composed by Āchārya Vaṭṭakera, usually dated around 2nd century CE (c. 150 CE). ( everything.explained.today)

So, Shatkhandagam is considered earlier/older than Mulachar.

You can read more about Shatkhandagam here