upgrade path for unlocked iphone with Sprint?

I currently own an iPhone 6S unlocked (bought full price) that I'm currently using with a postpaid Sprint account (Sprint SIM card installed). I need some advice on the right upgrade path to iPhone 8.

I intend to buy full price as before, so I'll get an unlocked unit. My understanding is the models sold for Sprint and SIM-less are identical since both cover the CDMA bands, but I'm not understanding the correct activation path. Is it:

* buy SIMless and have it activated to my Sprint account without any SIM card installed? OR

* buy and just transfer my current Sprint SIM card from the 6S to the 8?

Will either strategy work? Any advantages/disadvantages?

Thanks