To feel fullest, you definitely need the Duo 2.
This is my full review of the Inside Out from April 2021 and this is my full review of the Duo 2 from December 2022.
It’s quite interesting for me to read them back after all this time. I clearly did like the afterglow function when I first used it and perhaps I should go back to using it more but honestly, these days, I do, by default, stick with constant strong vibration and don’t bother with the afterglow. It is a quick worker and I love it for solo sessions. A year and a half on I would say the additional features of the Duo 2 are not worth the extra money but, for me, the girthier arm is. That is a personal preference thing but I like the bit of extra girth.