Enterprise does not allow booking on specific car models. However, if there is an Enterprise reservation, the branches (local or airport) will often pull cars from National or Alamo if nothing is available. Enterprise will pull whatever is available, and there is no difference between PXAR and PCAR to Enterprise.

Requesting the 300 makes your booking even more difficult IMO because there are just less 300s compared to Impalas at least in my experience. Car models are not guaranteed.

The only recommendation I can make is to try to come back later and see if they have another PCAR to switch into. As far as service, you might the same experience if you book with Enterprise. Swap-outs are much more difficult with Enterprise though.