Generally, ordinal numbers are namba + cardinal number. First as an option is socially marked as more ‘English’ or possibly more ‘educated’, but we need corpus/fieldwork to determine this.
Nambawan as an ordinal ('first') may also be increasingly blocked by its colloquial meaning of 'best'. The more natural sense of
Samting ia i nambawan is 'That's great', not 'That's the first (in a series)'.