Thanks everyone above for commenting.
Yes maybe I should give you guys more information.
I am going to expand my train set so I probably would buy extra tracks and maybe 1 more train set. My ultimate goal is to get the Emerald because it is just COOL! (of course power it too.). Also, I probably will get some more pieces from here and there to modify my own train(s).
In terms of playability, yes I agree with you all that the old cargo train has more playability. I want to get the new cargo train but it is quite expensive....and that's why I pick passenger train which is cheaper as 1 of my possible choices.
On the other hand, the new PF is nice as how you all mentioned it. And I believe the new set + new PF motor set will give us more flexibility in extending and supporting future trains/parts that LEGO is going to release right?
Hence, It is pretty hard for me to choose :( painful ~~~
Would it be possible to swap the new PF into the old cargo train deluxe?
hmnn...so I refined my choices:
1. the old 7898 Cargo Train Deluxe with old remote, motor and battery box
2. the new 7938 Passenger Train with new PF remote, motor and battery box
3. Get the Emerald Night + the PF motor sets separately
4. old 7898 Cargo Train Deluxe + PF modded ( buy separately ) <-- is this possible?
5. Get the Emerald Night + RC Train motor set separately <-- is this possible?
Thanks!