You might want to clarify what you want to get out of the trip to help figure out who would best to go with. You will get the cultural experience whatever but do you want adrenalin or to test your climbing legs?
I went to Mustang with Tangi from Enduro Mtb Nepal 3 years ago – it was an awesome trip. His background is downhill, and his trips are focused more that way, so if that is what you are after then that is the right starting point. Around Kathmandu and Pokhara we had some uplifts as well as riding up, in Mustang there was a fair amount of up on foot and on bike but nicely balanced with tasty downhills.
He had plenty of stories about meeting people who had ended up on trips that involved mostly fire road type riding and were feeling disappointed – so if looking at anyone else, ask some questions about the nature of the trip. That said, the Upper Mustang trip on the Himalayan Singletrack website sounds frickin’ awesome.
Also beware the risk of big mountain addiction. I’m just back from Mountain Bike Kerala’s Singalila Ridge trip, and as above, Mike’s trip was awesome too and I am sorely tempted to book up another trip with him. That trip (which is at the wrong time for you) does crisscross the Nepalese border but its quite a different experience from the rest of Nepal.
You can find pictures from the recent trip and Nepal in here:
https://www.flickr.com/photos/garyhill/sets/%5B/url%5D
In summary – JFDI.
EDIT – almost forgot, if you do go to Nepal, you really must make sure you see more than just Kathmandu and/or Pokhara. Obviously there are a huge number of things to see around Kathmandu but it is also hectic and polluted. It was a real shock coming back to it on the way home.