Gigabit Ethernet: Buy Built In or Buy Apple’s Gigabit Ethernet Adapter?

I’m thinking of getting a new Mac Mini. I already own Apple’s Thunderbolt to Gigabit Ethernet Adapter, which I am not currently using.

Am I correct in assuming that foregoing the Gigabit Ethernet port upgrade on the Mac Mini in order to use the adapter instead will produce the same, or roughly the same, Ethernet speeds?