Compare the terms before the prices. If you use the service regularly and can estimate your monthly data needs, a monthly plan makes budgeting easier. If your usage is intermittent, a data package that never expires may suit you better. The two options use different billing units, so comparing the price figures alone can be misleading.
If you're not sure how much data you need, check usage records on your devices and estimate separately for browsing, collaboration tools and video. Video quality, autoplay and background syncing can all affect consumption. Before submitting an order in your user panel, double-check the plan name, data allowance, billing option and amount due.