It doesn't matter. Just play what you enjoy the most since 4-man contents are easy enough to do so. For 8-man, just look for a group that needs the jobs you prefer to play or build one with your current 4-man static. You should get a PLD, WHM and 2 other DPS. If you decide to go BLM, best to get a DRG and a SMN. If you decide to do DRG, best to get a BLM and a SMN. But work with what you get, there's almost never a bad group composition. There are just bad players.