I'm guessing you may have a SoftLayer server (I could be wrong though).
A 100Mbit port would give you up to 100Mbit (12.5MBPS) in speeds.
A 10Mbit port would give you up to 10Mbit (1.5MBPS) in speeds.
The 100Mbit port is faster, but is limited to the amount of transfer you have.
I would recommend trying to figure out how much bandwidth (speed) you use (on a chart or something) on a week/month basis, if you use more then 10Mbit (1.5MBPS), you NEED to stick with the 100Mbit port so then you don't max out your connection.
according to that graph you do occasionally go over 10Mbps. You would have packet loss during those times if you went with 10Mbps unmetered AND were capped at 10Mbps...some companies allow you to burst and you just pay overage per Mbps based on 95th percentile.