Choosing the best codec/file format for importing into Premiere Pro

You don't have to convert, but I do recommend using Proxies for all such footage.  Stick to the included Cineform presets.

Work offline using proxy media |

FCP-X does the background conversion to ProRes because H.264 is a great acquisition and delivery format but taxes hardware - because of its dense, computationally intensive compression - to edit.

Converting the camera files to ProRes offers the advantages of working in a superior quality, robust, designed for editing format that is also very hardware friendly.

There is no image quality loss in converting H.264 source to ProRes, editing and exporting a ProRes master, and then converting the ProRes master to a delivery format.

The downside to using ProRes: the time it takes to do the conversion, and the disk space ProRes files occupy (typically 5 to 12 times the size of the H.264 file).

To speed up the process, much like FCP-X does, Premiere added an Ingest workflow that lets you do the transcoding in the background.

Adobe Premiere Pro Help | Ingest and Proxy Workflow in Premiere Pro CC 2015.3

we've been using FCPX here for years now but looking at moving to Premiere as we're starting to do intros and effects in After Effects, so presumably it'll be easier or better to have an all Adobe workflow for or video stuff.

f you are comfortable in FCP-X, you can export rendered outputs from After Effects in ProRes that you can import and use directly in FCP-X. Using After Effects does not require working in Premiere Pro. Many editors use After Effects with Avid, FCP-X, FCP-7 and other editors.

MtD

No need to convert.  Simply copy the contents of the card to your project folder on your hard drive.  Then in PPro, use Media Browser to import the video clips into your project.  Start editing. 

The only thing to add to Ann's comment, is that at times (especially with H.264-4k) it's useful to use the "ingestion" process in PrPro's Media browser ... and create proxies that PrPro switches in/out of with a click. I don't know why they included an H.264 proxy preset, when the main reason for doing so it to get away from long-GOP media for playback.

So ... for using proxies for easy playback, stick to the included Cineform presets. They process through for playback "like butter" as peer Jim Simons always says.

Neil