First of all, I have no experience with UV printers. But, I work with standard fluorescent lights quite a bit. The long-term reliability of magnetic ballasts seems to be much better than electronic ballasts. I realize that's a broad statement, but in general, our success with electronic ballasts has been poor. However, in your case, you probably won't be using the printer 8 hours a day five days a week for years, so reliability might not be a problem. Still, I have more faith in preheat magnetic ballasts than electronic.
Also, you have to watch the ballast factor. The higher the ballast factor, the brighter the tubes will be. Most ballasts should be around 1. Above 1 and the tubes are overdriven, under 1 and the tubes are underdriven. Overdriving the tubes will make them brighter but they won't last as long. Just something to keep in mind. If you are concerned about printing time, be wary of ballasts with a ballast factor of much less than 1.