Since I bothered to order my first PCB from two services, i figured I'd post a quick comparison for reference. First and foremost, I noticed that that the DirtyPCB board had my mechanical holes I specified, whereas the OSHPark board did not. I didn't realize this, but when I placed the pad to create the hole, I selected 'NPTH, Mechanical', in KiCad, which for whatever reason spits out that into a separate drill file. Looking back in OSHParks FAQ, their service will only read one drill file and ignore any others supplied without warning, so be careful with that one. DirtyPCBs apparently is fine with multiple files.

The second difference is color. OSHPark as you may know is the purple board. DirtyPCBs has options for a few colors, I chose the default red.

Another point is that DirtyPCB stuck a batch label on the front of my. Small and inconspicuous, but different.

The DirtyPCB boards also came with smoothed edges, whereas the OSHPark boards have the panel connecting nibs on them.

The last board difference I can notice is that OSHPark has the gold plated finish on the pads, whereas the DirtyPCB ones are tinned.

Finally of course, OSHPark is made in the USA and DirtyPCBs is made in China, which is a big difference in shipping times if you do the standard cheap shipping. DirtyPCBs had the boards made and shipped pretty quick though.