Depends on what you consider important but all three are much of the same.
If I was to start again I'd look at VED as that's something that will always creep up, and any car before 2017 will be on a collision course with £1k a year in the not to distant future. If running costs are a concern that is. As for fuel, the F range is kind on fuel considering the engine capacity.
Styling will always be a decider and the spindle grill is like marmite, and something that put me off of the newer vehicles. It's not my bag so I went with the ISF which was also kinder when it came to garage space. Perfect 4 door for my family and because I'm someone that likes to play with cars, a platform that has some pretty good support for modifications, mainly overseas.
The only issue is the fact that all three models are pretty much the same performance wise, the ISF being the lightest out of the three, including the RCF track edition, which is still heavier than the first ever F car! There's no head and shoulders performer, unless you modify them, which is where I'm at now with my ISF. I thought about chopping it in for one of the other models, but everything else would be a lot slower than where I'm at now with the ISF and they can't really do anything better than what I have, bar a nicer interior which seems to have delamination issues with its carbon trim! This is fundamentally why the F range is so unappealing to anyone outside of it. Lexus just don't push the boat out or go all in, they play safe and all the cars have kept almost identical performance figures over the years, whilst the competition have moved on.
I'd certainly look at the competition and see what's out there too. There's some good cars kicking around like the M8 competition, which for the price looks like an animal.
Good luck with your hunt.