PG if you don't have a PG sensitivity, VG if you do. Or if you like spending 20 minutes trying to shake treacle. Or if you want the very highest VG ratio in your juice. Your flavours will be in PG and total flavour % is usually anywhere from 6-25%. So if you absolutely must have an 80 VG/20 PG ratio in your final juice, you may struggle to get there with PG nic and PG flavours. If you favour a more standard 70/30, you should be fine with PG nic for all but the very highest total flavour % recipes - which are becoming rarer as total % trends downwards.