What are the differences in reserve positions? Marine, Army, Navy, Guard, etc... What do you recommend?

You want us to break down the differences between each branch? That's a massively open ended question.

I was Army (Active) most of my time and ARNG for 18 months for the last of it.
Infantry Unit for most which was a great experience. As a Medic, a lot of "waiting" for someone to get hurt, giving classes, constant medical classes, and training.
In the ARNG, it was 1 weekend/month 2 weeks/year. Most of the time it was the same thing and that unit didn't have many who had deployed before. I was tasked with giving my Active Duty experience as well as training medics on how a deployment would be.
I'm glad I got out before the current 'admin' and woke "Army".

That depends on what you want to do, for example, if you want to be on the ground working or fighting, then choose between Army and Marines. But if you want to become a technician, pilot, or something of that nature, I would say look into Navy only if you do not mind being in the ocean for 6 months at a time, or Airforce who are spoiled in the sense that they get what they need for the job. Just remember, all branches have great jobs, but some are better equipped than others. For example, the Air Force looks out for their troops in the sense that they build sponge-feeling sidewalks for the troops to run on. I hope this gives you an idea of how to pick good luck on your choosing.