Honestly a lot of the "Mil-spec, mil-grade, etc..." is marketing speak. Real military tubes do go through a specific testing regime which adds cost that most commercial users don't want to pay, and so you see a bunch of terms like "mil-spec, mil-grade" etc. Keep in mind that some of the tubes good manufacturers use are the same tubes the military uses, but they are typically then built up into a device by the seller and not the original manufacturer.
I believe 40 degrees (+/- 2) is standard. I've not personally used the 51 degree, but that would only be a few degrees on either side and would still be the same feeling of looking through a soda straw. In practice you will still need to keep your head constantly moving to use either one.
For companies, the tubes themselves are either L3 or Harris. Current gen are L3 Omni VIII or the Harris Pinnacle series. If you can afford it you should get either a Pinnacle or Omni VIII tube with a good SNR ratio. Many secondary manufacturers will use lower grade or older generation tubes. However the tubes are then attached to a housing, power supply and packaged into a device by someone like TNVC, ATN, OPMOD, FLIR, etc... and them some of these are also resold on outlets like Amazon, Optics Planet. etc...
As a result it can be hard to figure out what you are actually buying. Many will just say "Gen 3" tubes, which in most cases are not the latest Pinnacle or Omni VIII tubes. Also look at the warranty. Some well respected places like TNVC have started offering 10 year or even lifetime warranties, while others offer only a 1-2 year warranty. I personally like TNVC.
Finally look at the price - if it is a "too good to be true" price it is probably is too good to be true. A top quality monocular in green with a good warranty will probably run you at least $2800-3200 on sale, and white will run $600-1000 more.