Hi Flickerfly,
I don't have experience with either of the humidity sensors you're talking about. I went with the HIH-4030 breakout from Sparkfun. I just wanted to let you know that 5-10 sensors is probably overkill and a potential headache. Everything I've read about humidity sensors is that you shouldn't ever expect more than two sensors to agree about what the humidity is, even if their right next to each other. So having 5+ would be worse obviously. That is, if you want to get an "actual" humidity percentage reading. If you only care about reading that are relative then you might be fine with more than one or two. And by relative, I'm meaning that if you only care about humidity trends of up or down, and by how much. Hmm, maybe I'm still not being clear. Say sensor one is 50%, and sensor two is 54% and they're sitting right next to each other. If you put a cup of steaming water exactly between them they might both jump up to 70% and 74% respectively.
The other reason more than one sensor might be overkill is that my experience with this single sensor is that it's very sensitive. My sensor is through three doorways, down a hallway, and around four corners from my shower (about 40 feet total). But it still easily shows an obvious increase in humidity after every shower we take. It also shows an obvious drop in humidity any time the HVAC (heating or cooling) is active. It will even show an increase in humidity when it's raining outside.
For reference, my home is only 7 years old and pretty well insulated. I'm in Northern California, near Sacramento. And I've got about 2000 square feet, and the home is single story.
I do have 8 DS18B20 sensors throughout the house, and one outside. They seem to be the sensors I pay more attention to. But to each his own. I certainly wouldn't mind having multiple humidity sensors (including one outside), but it's currently something I wouldn't spend my money on.
I can't speak to the wire length question you have for the humidity sensors since mine is only a couple inches away from the Arduino. But the DS18B20 sensors work pretty nicely over the CAT-5E wiring in my house with distances up to about 60 feet. I wired them up using the existing wiring that the home builder put in for phone/network. Most of my rooms had a spare pair of wires in the CAT-5 so I used them in parasite power mode. The topology is star, aka. home run. I did have to create two busses for all the sensors to make it more reliable though. I think you'll be fine with your 50' run.
I've bought a second HIH-4030 to use outside, but haven't put it in yet. My plan was to put it inside a plastic outdoor electrical box to protect it from the elements, then put it under a roof eave. I thought that having a hole in the plastic box would allow moisture in, but also bugs and dust. So I thought I'd get a small bit of Tyvek house wrap (or envelopes) over the hole. Tyvek is designed to let water vapor through, but not water drops, or hopefully bugs or dust. A small bit of metal screen over the hole would keep out bugs that might be able to bite through the Tyvek. My box is also supposed to hold an Arduino clone (the Dorkboard) in it, so I want that extra protection. Two holes, or a larger hole might give better air flow.
I hope this helps. Have fun in what ever you end up doing!