Hello RyanJames,
First question: Differences...
* When you purchase P3D and start it up, you are in the default flight at an airport (large) in Virginia.
* You 'do not' have a boot up or setup screen, you boot straight into a flight. From there you can go into other saved flights, or set up a new flight. There is a small program you can download that has the ability to start up P3D in what ever saved flight you want. It works nicely, free download.
* Menu screens are different, all being that light gray blue color and light gray color.
* Stock planes do not include all the FSX planes. None of the FSX airliners, no Cessnas. You have the stock Mooney Bravo and Piper Cub, and will now have the Iris T6 Texan as well as a new, updated Mooney Acclaim in pure, new FSX formats.
* P3D has the ability to run 4096 ground textures. Some terrain is redone and looks better. Mountains seem to look better. The colors seem brighter and more rich then FSX.
* P3D is growing into the ocean segment and has a submarine and you can activate 'Bathymetry' which is undersea 'ground' (the sea floor) in selected areas around Earth. They plan to expand on that. Military training and research groups that make submarines and vessles are interested in using P3D for use in air/sea interactions. So that is in it.
* In Windows 7 64 and x86, you must run P3 in Administrator mode. Right click on the P3 'exe' icon and select 'run as'.... and click on Administrator. Otherwise it has issues saving some settings, etc.
* In Windows 7, you will probably have to hand enter DLL files into your DLL.XML main sheet. FSX has one also, and auto loads these. In Windows XP, it auto loads fine, no worries, in P3D.
* For me, the main, number one difference is 'smoothness'. I can run P3D with default settings, 20 FPS, full maxed out special effects scenery (buildings, but not autogen), nice water settings, real clouds, ground sea and air traffic at 15 to 20%, running all of these things on my single core computer and it runs like FS2004. It runs very nice. No super rig required. I 'do not' have Autogen maxed out, and I have been having problems running ORBX PNW as it fills up my graphics card V-RAM and crashes the sim. So scenery can run out or max out your video ram (graphics cards). Some flights in PNW with 'lighter planes' will not max out the V-Ram and long, 2 hour flights are fine in PNW. Heavy planes with maxxed out, giant textures with rich PNW scenery will make it difficult with basic, low RAM graphics cards, as like it would with FSX, (same thing. In FSX, your planes start turning black, then crumpling up, then things start malfunctioning. P3 will continue to run smoothly as long as possible then bang, it falls over. This takes a "LOT" to do though. Its a maximum graphics scenario).
Second question: What can you bring in....
Everything FS2004 and FSX. Most all scenery works. Some people place the FSX.exe file (a copy) into the P3D folder if the installers will not install into that folder (missing FSX file). Putting that in there helps to successfully install tempermental installer packages into P3. I have migrated A2A, Carenado, and my own works into P3 fine. They fly awesome and smooth in it.
ADDITIONAL NOTES: You do not need the FSX SP1, or FSX SP2 updates. This is a fully updated 'new' simulator that was 'once' FSX ESP. You buy and download a uptodate sim. No updates. Build 1.4 will come out on Monday. That will be the highest version, and you will have all that you need in it to run it, nothing to update.
This has the files for Accelleration. It does not have the FA-18 jet fighter, and it does not have the Mustang air racer. It has the ability to land on carriers, arresting hooks controls, etc. But not the Acceleration planes.
It has some aircraft donated by JustFlight for additional aircraft, such as the classic Lockheed Constellation airliners and the Lockheed P-38 twin fighter from WWII.
I hope that helps.
Bill