National mobile - 300 texts for $9.99
airtime-500 texts for $17.99
which is the better buy?

For the national mobile, you have 300 texts for 9.99 dollars whereas for airtime, you have 500 texts for 17.99 dollars.  

9.99 dollars/300 texts = 0.0333 dollars per text
17.99 dollars/500 texts = 0.036 dollars per text  

Choose the cost which has the lowest rate. If you really like to travel to other places and you need to budget, you will choose the national mobile rate. It is cheaper and 300 texts is a lot. However, if you want to travel including using your mobile texts during air transportation, you need to buy the 17.99 dollars. It’s a bit expensive but the number of texts is a lot. Divide the cost by the number of texts. For costing purposes, its smarter to choose the national mobile rate.