If possible... See if there's a spec listed for both that gives the kV (kilo Volt) rating for them.
Ideally, you want something that's in the vicinity of 15kV or more.
I've built several over the years, power output is king. The more the better. My most recent iteration is a 30kV unit, based around a commercial DC negative Ion generator. The downside to such a unit is that they're $50, which isn't too much of a downside unless you let the smoke out of it by wiring it wrong or putting too high an input voltage into it (which can happen).
I brought the cost down by buying 6 to get a quantity discount, which I think brought the price down to $35 each. But it only made sense because I was going to build a few for some friends and my father in law.
Jeff