I own both of those cameras, and I know how you feel. The D90 is a very capable camera if you choose to pursue it, and of course there's a selection of lenses available for it. It is certainly more work and a steeper learning curve than the Sony. There is a very good book by Darrell Young called Mastering the Nikon D90, it may be of interest.
The Sony RX100 line is very compelling, in particular as as travel camera. Excellent optics, large sensor-that's what get's it done!-I absolutely love my base model, and would consider leaving the DSLR at home for travel. Having said that, I would love to step up to the new-ish D500 body and use the several good Nikon lenses I have; that would not get left at home!
If you are after a highly capable, easy to use camera for travel, there is no better choice imho than one of the RX100 models. Are you able to handle and operate the used one? The base RX100 is usually around $449 new, from BH Photo, for example. Personally I'd be hesitant to buy that used if I was not able to see and touch it and evaluate it. Good luck!