What advice do you have for someone reclassifying to 15T? Should I go NG or stay active?

I can't comment on the transition from 11B to 15T, but can give you some advice on the rest.

The latest In/Out Call message* from HRC doesn't show an 11B to 15T transfer is possible, so you'll be limited to do it at your reenlistment.

As to the "Active Duty" or "National Guard" ... there a LOT there to unpack. There will be a completely different dynamic to consider between the two (not the least of which is what you'll be doing on your civilian side - job? back to school? etc.). One resource I would recommend you look at if you're considering transferring to the reserve component is a guide on Military OneSource*.

The majority of your "non-annual training" flying in the reserve component will happen during your "Additional Flight Training Periods" (AFTPs). AFTPs are additional inactive duty training (IDT) authorizations that aviation personnel are authorized to use .. but like everything, authorization means "you can, not you will" as it is heavily dependent on funding. For both the USAR and ARNG, the AFTPs are capped at 48 a year which translates into 24 days (an IDT is no less than 4 hours long and you have a max of 2 each calendar day).

So, barring additional active duty periods, the amount of "flying time" ON AVERAGE will be about 30 or so days in the reserve component when you combine your AFTPs, miscellaneous flying time and training flights during Annual Training (some units may have more time and some may have less ... much will depend on funds available as opposed to training or missions available).

As far as your comment, "Reason I’d go NG is because I’d possibly stay E4 longer to give me a greater chance at crewing", that's pretty much a red-herring based on the short amount of time an E-4 spends before he meets the TIG requirements for promotion. I guess you could always ask your commander NOT to recommend you for promotion, but regardless if you are recommended or not, you'll not spend a lot of time as a SPC unless you purposefully make yourself ineligible for promotion.
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* https://www.hrc.army.mil/milper/23-033
* https://www.militaryonesource.mil/military-basics/career-advancement/changing-mos-jobs-in-the-military/#:~:text=You%20can%20request%20a%20specialty,move%20to%20a%20new%20position.

15T. I would suggest you research the NG units near you if your thinking of that route. As active you probably have a higher chance of actually working on a helicopter. If you go NG and don't research the facility you could find yourself just posting changes to manuals every drill weekend and not even touching a helicopter...

Compared to infantry, it is a much more technically demanding field. I loved working on my Blackhawks. I never felt that I had overcome the learning curve though even wirked in QC and ran a phase maintenance team. I also only crewed occasionally. None of my peers held that against me as long as I was pushing to learn more about the airframe and be a SME. As for Gaurd vs Active, I was active I did reclass with a young man that would be working as a civilian mechanic for the state when he was not on duty. Which probably paid well. I don't think he had the opportunity to fly as much as I did though. So the answer to your last question is yours. What do you want? And i flew quite a bit as a team lead and TI, so don't let the rank tie you down.
Good luck,