I'm only going on my impressions from having seen threads across various boards (not very empirical a measurement, but then neither's this poll). Most people are very happy with their Vortechs, very, very happy. But there seemed to be marginally more (i.e. a slightly larger very small minority than the Tunze very small minority), who had problems with their units. Probably not worth worrying about if you're in the US, as you can simply send the unit back or get Vortech to send out a replacement part (their customer service in the US seems good), but in my particular situation I wanted to reduce the small chance of something going wrong (even if it's reducing it from 4% to 2%). Particularly after I saw a thread on a Vortech forum somewhere where they told a customer who'd purchased a Vortech unit from the US but was based overseas that they wouldn't help him and he wouldn't be covered by his guarantee with the local distributor wherever he was based.
All not particularly relevant for the majority of people, I'd agree, but it swung me. Tunze also make a fine product and I decided to go with it.