Arduino Robot - what to start with

Been playing with Arduinos the past year for home automation IOT stuff. Also wanted to try out some robotics. After watching BattleBots last few weeks my son has been wanting to build a robot with me. So I want to get on this while he is interested.

-start with line following or obstacle avoiding robot -will want it to do something more than drive around eventually -remote control via an ipad and android apps would be a huge plus at some point, but don't just want a RC car

Looking for cheap chassis that will work and allow for lots of upgrades/sensors (not sure what to really start off with). So I came across these two items, thoughts?

Small two wheel chassis ... Motor Driver shield ...

Or just go all out for something like this which has arduino, bluetooth, sensors and chassis: