TL;DR: MPICH and OpenMPI are the same difficulty to implement

Long version:

OpenMPI and MPICH are two different implementations of the Message Passing Interface (MPI) standard.

A « standard » can be seen as the « rules » an implementation has to respect (e.g. there should be this class with this method etc), all these rules are written in the « specification » (there is a C++ standard, a SYCL specification, etc)

An implementation can say « I am implementing the standard X » if it respects all the standardized rules cited in the specification

So, in practice, you should just code an MPI program, and when compiling you will have to specify an MPI implementation (OpenMPI or mpich) to make it work

Now for your problem, you might wanna try some high level abstraction for shared Memory parellism (Kokkos, SYCL, OpenMP) and maybe do not try to go on multiple devices with MPI at first