I took an F Sport for a drive and found the suspension too stiff, so I ended up with the Sport design as I wanted the softer ride yet have the more aggressive front end.

 

The F-Sport seems to have a suspension that is too stiff for some people. This is more evident in parts of the country where the road surfaces aren't particularly smooth. Find some rough roads and take both of them for test drives over these roads on the same day. That may help you decide.

 

 

IIRC, the sport design is just the regular RX350 with a different front bumper and the F-Sport rims I believe. The F-Sport has actual differences versus the regular model like the 8 speed with paddle shifters, some different interior color choices (I believe red was offered at one point) and design differences (like sport pedals, leather steering wheel only with grips), suspension differences (harsher versus the regular model), and different rim design. Only offered in AWD as well. It's a relatively rare package to come across for this gen unfortunately. Back when I was looking for an RX a few years ago, I was looking for an F-Sport near my area and could not find one, so I settled with what I found.

If you've ever driven one of the newer RX's (16+), it's the same 8 speed transmission. It can be a bit jerky in stop and go, and I noticed it tended to hunt for gears when I drove one. Although, that seems to be relatively common among all 8 speeds as my Yukon does the same thing (although it feels less noticeable than the RX's did).

 

As Zmon mentioned above, F-sport only comes with AWD (Real world MPG on highway is 21.5 & 18 for city). We purchased the F-Sport version 3 years ago because it looked very aggressive (comparing to the garden variety version) and came to realize that it was really uncomfortable driving around town as the suspension was way too stiff (way worse for back seat passengers). In the hindsight, I would rather choose the Sport Design version over F-Sport.