Both are DIY systems built on top of the same reverse-engineered (i.e. hacked) communications to Dexcom CGM and compatible MM pumps. Both require some work to setup.
They are different, and you do not need both. In short, the two setups are as follows:
OpenAPS = (code that runs on a small computer board*) + (pump communication device*) + (CGM) + (compatible pump) + ((highly recommended) Nightscout setup) + ((optional) watch)
*several different options are available for these components
Loop = (code that runs on iPhone) + (RileyLink**) + (CGM) + (compatible pump) + ((optional) Nightscout setup) + ((optional) watch)
**RileyLink is a small custom board, which can be purchased separately, and which works out of the box - no special skill required.
Most people use Dexcom CGM (G4+share, or G5), but the systems can also be put together using Medtronic Enlite CGM.
Here is a basic diagram that shows what one would need to setup Loop or OpenAPS:
You cannot buy a new compatible pump (with a possible exception of Dana R pump sold in Europe). An option available is to look for a used pump among the MM models shown in the diagram.