unpaid
I would not recommend stealing one either.
Are you going to build your prototype on breadboard, at least initially? If so, I recommend an Arduino that is breadboard compatible. Uno and Mega are not. They are meant to be used with shields, not breadboards.
To recommend which "Arduino", we need to know how many ws2812b LEDs your final project will use. Ram memory can be an issue with high numbers of LEDs.