Depends on what you usually use. If you usually use a fixed voltage PV, it doesn't make much sense to test juice with a variable voltage when you're sitting home. A juice that tastes good with a 3ohm atomizer at 4.8 volts, might not taste so good with your 3.4V ego and a 2ohm cartomizer.
A battery just supplies power. Match what you can achieve with the PV you use the most. If you use a 3.7V unregulated PV most of the time, then get a 3.7V unregulated PV to test juice.
Don't get a Twist to test juices unless you're going to be using a variable voltage PV when you're not at home. That makes no sense unless you only set it at 3.7 or 3.4 volts, which also makes no sense. It's like testing winter jackets on a beach in Florida in July.
And test with an atomizer that's the same resistance as the cartomizers or clearomizers you use on the PV you use most often.