I don't have the Flex any more (a BCer contacted me privately and asked to buy it off me) but, from memory, I think the Fifteen has more tonal variety (including helpful LP and HP filters) and can be driven a bit harder. Its also quieter in operation. Early impressions of Fifteen are that its exactly the preamp/DI pedal I need.
Edit: I also had a nagging fear over valve failure with the Flex as I previously owned an EBS Valvedrive which died on me (yes, its easy to replace a dead valve but not during a rehearsal or gig!)