A few guys from work have been using Nitrogen for a while now. They say that tyres hold pressure for longer and also claim better mpg.
Price?, one guy got his done for free at quick-fit and another was charged £1 per tyre. Worth a try??
As for tyres,I had the Re070's in 17" and hated them,car tramlined all over the place.Have used the Toyo t1's in both 17" and 18" and love them.Had them on right through the winter and even made it up single track country roads to the stables when the Disco got stuck the previous day.
Hope this helps,
Colin.