This can actually be quite complicated to work out and sometimes it's just not worth the effort of trying to find the perfect option. However, there are some basic considerations:
Are you on an older device-inclusive tarriff that has long since matured? If so, consider SIM-only.
As has been said previously, are you continually bumping against your data limt and needing to buy extra?
Also, are you regularly well within your limit (you can look at past bills or consult you phone's data monitor) and would a smaller allowance be better value, even if you occasionally need to buy extra? All tarriffs now include unlimited calls and texts, so we'll ignore those.
Do you ever use your phone abroad? If not, and you have no intention to do so, would a Basic tarriff be better (these have some serious limits, so check carefully)?
Finally, remember that, during the initial period, you can only upgrade a tarriff, not downgrade it (this doesn't apply after the minimum period), so it's better to start a little too low than too high if you're being cautious.
Edited to add: another thing to watch is the annual price increases. Once you've completed the minimum term, you can usually go back to the base price by simply upgrading to the same tarriff if you're happy to stay with the network.
