There's a few things that vary on the RX's over the years. I dug into this a bit when we were looking, might be handy for you. A lot of times ads would get the trim level wrong
We like the park sensors/blind spot, and the heads up display is very good
3M on the hood is also useful, as this gen can chip pretty badly without it
2014
Regular Base
Not sure if there is a lot of these
Regular Premium
Adds heated and cooled seats (which are awesome) drivers seat memory, sunroof, etc
Believe both this and the base have the simpler non mouse based control system, smaller screen
Regular Touring
The above plus parking sensors, blind spot monitor, heated wheel, navigation
Regular Tech
Adds Mark Levinson sound, heads up display, thigh support
Fsport series I
8 speed, slightly harder suspension, dark wheels, aluminum trim inside, 'chin' front
Fsport series 2
The above plus heads up display, park sensors, likely the Mark Levinson
2015
Sportdesign
All have the bigger screen now, moonroof, heated/cooled seats, 'chin' front
Touring
Adds park sensors, blind spot monitoring, heated wheel, nav
Tech
To above adds Mark Levinson, Heads up display
Fsport
All are series 2 now with Mark Levinson, blind spot, park sensors, heads up