AN arpeggio is a broken chord, so C Maj (C - E - G) will only be played from the notes of a chord in any direction (up, down, inside out etc), also an arp in synthesis terms can play through one, two or three octaves usually and at different note divisions etc.
AN Ostinato is a musical phrase or sequence which is not confined to a chord and has a melodic intention (doesn’t rely on a chord). A 303 riff is actually an Ostinato, An Ostinato could be C, F, A, C, E2 etc etc
On many occasions they can be exactly the same thing coincidentally. That’s how I interpret it anyways