trashbot v4, 9dof biped, controlled by arduino nano

over the last eight weeks i had rarely time to work on it but still wanted to share this with my beloved community. i changed the chest to house two batteries and added a 3 dof neck and all of this changed the weight distribution so that the PID controlling the spine has to be retuned. i'd love to have a more structured approach in finding the right parameters as this is really hard work. finally, i'll need to upgrade the hardware soon if i want to add arms. the nano right now will not have enough pins to control the relay, ultrasound and the servos. the choices are: second nano via i2c (nice and easy, but low computing power) or change the nano to a teensy3.1 (best computing power but hard to connect) or an arduino due (mid range computing power, easy to connect but bulky). what's your opinion on this? ps: the full construction of this version is on my blog: http://assadollahi.de/trashbot-4-and-more/