Choose unlimited if you do not want to watch a counter. There is no total allowance to run out, so a phone update or a photo backup cannot leave you offline outside the airport.
Choose fixed data if you know roughly how much you need and want the smallest possible price. A light week of maps and messages can cost less on a small fixed package.
The difference that matters on an unlimited plan is the daily full-speed amount, not the total: every unlimited plan states how much runs at full speed each day and what speed it continues at afterwards.