I've used thick foam pad (comes with 7200bx) and thinner 3M "gel" tap (comes with ikon). Both work fine, at least for me. In general the thicker it is the better at lessening high frequency vibration transfer it will be. But it will also tend to soften the response to abrupt cyclic and tail changes.
Some have strong opinions on this topic, usually borne of having suffered some kind of vibration related problem which made the FBL system go nutty at the wrong time. I have yet to encounter such an issue, so my opinion is it doesn't really matter which you use.
More important is to make sure the FBL is perfectly squared up to the flight axes. If it is slightly canted one way or the other it can result in rolls injecting some elevator, or flips injecting aileron, or piros injecting one or the other (or both), etc. Unless it is skewed a lot the effect should be small and barely noticable, at least until you spend some time flying it and can notice the difference between what you're putting in and what it is doing on its own.
The only thing that makes me chose one tape over the other is what it is being mounted too. On the 300x, for example the stock mounting location for the 7200bx is inside the frame and the surface it mounts to has cutaways and whatnot. In that case the thick foam makes a lot of sense because it will allow you to snug down the 7200bx to the uneven surface. On the 550X, on the other hand, it is mounted to a plate and could as easily be mounted with thin foam or gel tape as with the thicker stuff. In those good mounting locations I like to use the thinner stuff so the unit is more tightly coupled to the frame/heli and I'm not having to make up for a bad mounting surface.
