Technique-wise, flute and sax are totally different even if the fingerings are similar, so I'd say clarinet would give you a better start for the sax - plus, sax is easier in many ways than clarinet with technique, etc, so as you are good on the clarinet you should be able to take up the sax easily.
as to which one to learn, depends mostly on which one you like best.
try them both out if you have a chance... see what you like the sound of, which one you find easiest to get a note out of, which one feels most natural to you.
listen to really good recordings of all both, what grabs you? which sound will inspire you most to practice?
my best of the 3 is definitely flute, and I can't pretend expertise on any of them, but... soundwise, probably sax is easier to get a note from, at least initially. flute fingerings and sax fingerings are both generally a lot simpler than clarinet - no going up a 12th when you hit the register key (yikes!). flute uses the most air because there is a lot of "wastage" (because you're blowing across the hole), but sax and clarinet require more support and feel like they use more air!! flute has a quite pure sound whereas clarinet sounds "denser", and sax is quite brassy - it's almost a brass instrument.
as a clarinet player, you probably won't have any problems with the sax at all, except maybe with adjusting to a simpler fingering system. and you probably would use a bit more air.
flute technique is quite different from clary. you'd have an advantage in breath control, and fingerings are similar for the first 2 octaves to the fingerings in the octave above the bottom register (similar to descant recorder fingerings). it's got quite a different sound and feel to it, - you would be making yourself more versitile - but probably more challenging for a clarinetist than the sax would be.
anyway... HTH