Chain is a bit heavier but has a bit more tolerance to misalignment to belts
ratio is easy - number of teeth sprocket a / number of teeth sprocket b

You need to calculate the sprocket distances carefully if you want to avoid a tensioner - assuming its a relatively short run
Chains will have a bit of stretch - give some ability to tension the chain e.g. slotted holes for the motor mount

I learnt alot with Spur 2 on sprockets and chains, it didnt put me off be makes me think a bit different!

Have a look at
http://www.revrobotics.com/content/d...cket-Guide.pdf