Lately, I have noticed that I may be missing out on not running Directx10 with FSX. I never ran it because the interface changed, not to my liking. Is the advantage significant with Directx10, if so what are the advantages?

The overall concensus is that it's best to NOT use DX10. A lot of 3rd party developers don't support it and most users don't use it. The main differences are that water looks a bit different and for some people the game runs better. However, with a bit of tweaking DX9 will run better then DX10. Disadvantages are that some things don't work anymore (progressive taxi), you may get flickering runway textures, and as said: some addons don't work with it.It's best to leave it alone and stick with DX9. I think that about 95 to 99% of the users here will agree.