Most likely the internal.
The Pegasus maxes out around 1000mb/s read, that's what a promise support guy told me in an email.
Bare feats benchmarked one with ssds inside which was faster. I wrote him an email what about hdd speeds? His answer:
"The fact that four of the fastest HDDs top out at 800MB/s means thats as fast as this 4 bay can go with HDDs.
However, a 6 bay or 8 bay TB 2.0 Pegasus could reach the 1300MB/s that we saw with four SSDs.
I hope to get one of those with more bays so I can illustrate that.
Rob"