I only listened to the Manley briefly at the '09 CanJam. Not long enough to form an opinion, but I had zero indication that it is a bad amp.
The Zana has been my go-to amp for three years in January. I can't say anything that hasn't been said before, but it is superb, fairly priced and dead reliable. It also makes an excellent preamp if you want to run speakers.
If I were you, I'd make the choice based on sonic signatures. The 300B is well-known for its sound. You'll get the magical mids DHT amps are famous for, with the highs a little rolled off, bass a little soft, and a more relaxed presentation.
The Zana is not anything like that. It is fast, authoritative, the bass hits, and it has bandwidth out to something like 60kHz. It is about as close as you can get to solid state while retaining just enough tube sound to let you know there aren't any chips inside. If you want accuracy and speed, this is it.
Also, you'll find that the 6C33C tubes are eminently reasonable, unlike the 300B. There's also a variety of 6SL7 tubes to choose from.
I'm not taking aim at DHT tubes, here. There's a reason why the Si2A3 is next to the Zana on the shelf. When I want a lush, magical presentation, the 2A3 tubes deliver.