What are considered valid reasons for switching from the Reserve to National Guard?

State your reason for wanting to switch - It's your dream MOS - back it up with supporting points, like career development, parallel professional/educational development, would be beneficial to the Army because its hard to find qualified people for 35F; whatever your case maybe.

Whether or not 'they' decide to let you transfer or not would have to do with the time you have left on your current enlistment and if the MOS that you want, 35F, is offered in a Reserve unit relatively close to you. If its not offered or is not close by...and you have a Guard unit that says they will accept you and have a slot for you (in a memo signed by the Company CDR or Readiness NCO at a minimum) it should be fairly easy. A Guard recruiter will help you if you are serious about it. Good Luck!

Having been in both, the National Guard feels more like family to me than the Reserves

Good reason is career advancement but when the Guard release you why you won’t go ACTIVE instead if thats your dream job?

Going 35F is a reason and you could explain that as well, needs of the Army etc, but like others have said career progression and rank advancement. You're going to have to get with a Guard Recruiter and your first line and explain your situation before it goes to your CO.

I would name a few things:
1. Career advancement (i.e. training and promotion)
2. Development (i.e. learning a valuable skill that has useful private sector applications)
3. Value to the military (i.e. there is not a large population of service members who can qualify for a MOS like 35F; you are doing the military a service by going into a position where numbers are needed)
4. Education value (i.e. the training for 35F is far more valuable and transferable to a civilian college degree than that of a mechanic)

I'm sure you can come up with a few others.

Better career progression and rank advancement, for starters

PFC Alberto, The Army Reserves will try to hang on to you as long as they can (unfortunately). Something to consider is how long you have left on your current enlistment. Go to the NG unit you want to join and get the Co. Commander to write a letter of acceptance. The NG recruiters will work with you to get you in. ultimately the USAR will release you, but they'll drag their feet. As far as reasons for your request. Don't bash the reserves, and focus on your needs and future. The Guard and USAR are in a struggle for every qualified soldier. you might mention that you want to serve your community and state during natural disasters or "State Active duty". A duty unique to the Nat'l Guard. Finally, Keep your business to yourself your peers don't need to comment on your future and keep plugging away until the USAR release you or you get to the end of you enlistment. Good luck!

The Reserves is mostly training and service support units. The Guard is mostly combat and combat support units. If you want to be closer to the front - join the Guard.

Looks to me your reasoning should be based on the MOS you are seeking and what is available. I know in my area, some MOS's are hard to find in the Reserves without driving a great distance and some MOS's are hard to find in the National Guard. The big kicker is what does your contract say, is there a bonus tied to this contract how much time was left on that contract. Some units are easy to work with when it comes to transferring and some are like we will get to it when we can.