I use pg base and mix at 70 30 as pg keeps better than the vg. It does depend on what strength your mixing at and conentrate percentages though as most concentrates are pg and can impact the calculations. If your mixing at a strength of around 1mg or less pg is the best bet
Tbh it all depends on your concentrate percentages. VG is best to go VG heavy, but PG keeps better. Try and buy small quantities that you know you'll use within a few months, or freeze any that you don't feel you'll use up quick.
I was told by a nic supplier that vg and pg nic both keep well if frozen in glass with minimal air gap. The most important thing is the quality to start off with.