The only digital cable that really matters is DVI-D for higher framerates. Some can't get past 60Hz at 1440p, but many can display at 120Hz.
There are some benefits to DP vs HDMI. DP supports freesync and g-sync, it has a slightly higher bandwidth, and has a locking mechanism built in.
Really though, you're right. A $3 cable is as good as a $100 cable. The beauty of HDMI is that the cable never changes, so there's no need to replace them.