i'm currently considering the exact same setup
decided on this one with it being well priced, but also adjustable height which i wouldn't mind trying with having a very low bumper and also being able to have it on SOFT for general driving & then HARD with the press of a button for when i feel like having a play or when it goes to Elvington etc
the Flex seems significantly more expensive but for what i'm not sure, they appear to only be pillow ball upper mounts, which i've read isn't necessary & other than that the text just appears to say that the height & damper adjustments are completely seperate, but i'm not sure what advantage this gives
MKIVStore currently have a very good deal on at the mo, which includes free fitting of the Super Streets