Unmetered you have a capped connection speed e.g. 10Mb/sec - the provider blocks packets if you go over the cap so this option is fixed price.
Metered you (normally) have an uncapped connection speed (except for hardware limits but usually at least 100Mb/sec) but a limited number of gigabytes transfered per month. If you go over that limit you are billed extra.
Go to the forums for whatever game it is and find out how much bandwidth it typically consumes per player inbound+outbound, then multiply that by the maximum number of players you expect on your server and add a bit of safety to avoid lag - say 50%. That is the unmetered bandwidth you'd need for the game.
Then work out the average number of players you'd expect on the server over a 24 hour period and multiply that by the per player bandwidth and by 3600 seconds x 24 hours x 30 days, then divide by 8 to convert bits to bytes - that is the GB per month you need. Then add a safety margin on to avoid having to pay surcharges.
Now see which is cheaper from the providers you are considering.