What advice do you have for a prior-service Marine entering the Army?

Prior Marines generally either do exceptionally well, or exceptionally poor. The best thing you can do is immediately remove any chips that remain on your shoulders, forget all of the Marines "we're better than everyone" brainwashing nonsense, and get ready to adapt to your new environment. Listen a lot. Learn a lot. Talk very little. At least at first.
Start reading Army Regulations. Start with uniforms, awards, customs and courtesies, NCO guides, etc...
It's gonna take you time to get your records straight, so be prepared to be patient. Most S1's aren't used to prior Marines, so you getting your awards, military education, NCOES, etc... straight will be an adventure. Stay patient.
Don't rush towards promotion because you think all of your Marine experience makes you ready to be an Army NCO. It doesn't. Before you rush to the board, make sure you truly understand Army basics, counseling, awards, evaluations, promotions, your MOS, service and programs, uniforms, leaves and passes, all those thousands of things NCO's are expected to know. Remember, it's better to be a high speed E4 than a POS E5. Make sure you're ready to be an Army NCO.
This might seem negative in tone. It's not. I made the same transition and it was the best thing I ever did. In the Army, you'll have a better quality of life, better programs, more education, more training, more varied responsibilities and assignments, more diverse bases and units, and a real opportunity to shine. But remember, you joined the Army. You must adapt. If you don't succeed, the Army will replace you faster than you can walk out the door. Make sure you set yourself up for success. Find another prior Marine, preferably a Senior NCO or Officer for mentorship, guidance and advice. They'll understand the transition. Don't latch on to a junior prior Marine. It remains to be seen whether they'll sink or float.

I liked working with former Marines. They were always more even keeled and reserved with their emotional reactions. They got their tasks done without griping, and almost all of them kept Soldiers motivated without blowing up.

Prepare to slow down (so to speak). I have worked with the Marines during my time in and most of the time they seem to be to perfect, that being said never loose what you learned in the Marines use it to your best advantage. You have already learned basic Soldiering skills so now you have to learn how to use them to your advantage. PT is going to be one of the big Little things Exercise are different but produce the same results, Put on your learning head gear everyone you meet is someone to learn from. Now watch those Junior Soldiers that want to help to much most have different idea's of what works the best. Stick with your Staff NCO's and never be afraid to ask a question. Marines Find the Army a little lax, I guess is what I am trying to say so use your High speed low Drag energy and learning traits to enhance your Career. Give yourself plenty of time to learn before worrying about Promotion. Never discard what you learned in the Marines just put it in a different folder so you can pull it out for use later. Good Luck and welcome to the Army.