50VG/50PG or 70VG/30PG?
Most 10mls will be in a mix of 50% VG (Vegetable Glyerin) / 50% PG (Propylene Glycol). This is a thin consistency which is suited for low-powered pen-like devices. The thinness allows the eliquid to vapourise without clogging up your coils.
70VG/30PG eliquids are thicker and allows you to vape it at a much higher wattage. The higher wattage and increase in vapour gives you a much better flavour profile experience, however, you need to have a more powerful ecig to be able to heat the eliquid up enough.
So if you have a pen-like device like an Aspire PockeX or an Innokin Endura, you will need to use the 50/50 eliquids for the best performance. If you ecig is more powerful than those, you can use both 70/30 and 50/50 eliquids. Please keep in mind that 50/50 eliquids at a high wattage will vapourise very quickly and you will need to ensure that your tank is always full to avoid burning your coils.