Which is the better option while serving active duty, taking online classes or in-classroom classes?

Depends dude. Are you at a duty station and job that affords you the rare opportunity to take physical classes? When I was at MEDDAC, I was able to take physical night classes because we came in at 0430 and got off by 1500. Online classes are the easiest for most soldiers to stick with because you can do them from anywhere. I was at NTC once and submitted a final from the top of an FLA on a mountain.

Depends on many things. Assuming the schools are accredited and the programs themselves are accredited for the areas they need to be, you'll likely need to do both. Figure out what works for YOU.

Some people thrive online. I get frustrated. Thanks for the pile of books, I'll figure it out myself.

There are lots and lots of options now. Seems to me there should be education counselors at your local education center that could go over programs available to you both classroom based and online. I attended lots of colleges (Florida Southern, USF, UNF, FSCJ, UTSA, HPU, and then finally graduated from UMUC), but UMUC had accelerated sessions online which would have been nice to have had back in the 90s when trying to fit classes in between field problems. Anyway, I would ask the Ed Center if they have any counselors to discuss the options (from various schools).