Let's Make Robots!

Choosing a transistor

I've used two types of NPN transistors in the projects I've done so far and I've tried to investigate why, but my conclusion just doesn't seem to make sense (actually, I'm not sure if I've reached one...).

For use with an Arduino, I've used a PN2222 transistor:

 

Whereas, for BEAM solar bots, I've used the 2N3904:

 

Mr Basic motors vs Tamiya Twin Motor Gearbox kit

Hi,

I'm looking for advice as to whether the Mr Basic motor/chassis kit (http://www.robotgear.com.au/Product.aspx/Details/572) is worth its bargain price - or is would it be much more reliable to use the Tamiya Twin Motor kit ( http://www.robotgear.com.au/Product.aspx/Details/440) and build my own chassis etc?

And I've seen videos where direction changes of robots built with this kit skid the back 2 wheels - would it be an easy enough/recommended to replace the rear wheels of Mr Basic with a caster?

Thanks.