If you are on the $35/2.5gb plan or less and have 10 days or preferably 7 days or less remaining choosing the $15/1gb data add on and using it sparingly might be the best choice. Any unused data will remain on your account to use to supplement your plan data the next time you run out. You should consider scheduling a plan change on your next renewal to a higher data plan if you use more than 500mb of the data add on in your remaining days before renewal.
If you have used 2.5gb in 14 days or so you should change your plan now to the the $40/5gb plan or higher as you have much higher data needs then you originally anticipated.
If you are on the $40/5gb plan or more it most economical to bump up your plan to what suits your needs and change your plan now. The exception to this rule would be the $50/8.5gb as there is no higher plan....in normal circumstances adding funds and sending a support ticket requesting the moderators renew the plan early would be in order. If you are close to renewal due to long wait times for support atm the purchase of the $15/1gb data add on might be a better choice if you really need data and now!
You should also review your data usage. Set your phone's data counter, set your data warning and limiter to better track your usage. Use your data saver and turn off all background data this wastes up to one third of data on apps doing nothing for you in the background.
