Whatever they have open training slots for around your estimated starting date.
Latest additions to the seniority list have been a complete mix of FO Embraer Cityhopper, FO 737, FO A321N, SO 777/787 and SO A330. All those positions have the same basic pay, but obviously different schedules.
If you are lucky they may offer a choice because multiple training slots for different types are open, or you just get assigned one. You can bid for another fleet as soon as 6 months after joining I think though (it might be 12 months, not 100% sure).