Hi geremiaunit,
No not at all. I don’t make any specific recommendations because there are too many unknowns and variables involved (see mattress firmness/comfort levels in post #2 here). I only recommend that each person makes the choice that they believe is the best “match” for them in terms of PPP (Posture alignment, Pressure relief, and Personal preferences) and all the other parts of their personal value equation that are most important to them. This would be especially true when it comes to preference issues that have very little to do with being “better or worse”. There is also more about the most important parts of the “value” of a mattress purchase in post #13 here.
I don’t think that they were telling you that it would be a “waste of money” as much as that in their experience they believe that there may not be “enough” difference between them to to justify the additional cost in your case. The top layers of a mattress (usually the top 3" to 6") make the biggest contribution to how a mattress “feels” (depending on body weight and sleeping positions) and since these would be the same in both cases (the top 3 layers or 9" would be the same) your body weight may not be enough to notice a significant difference with the additional XF layer on the bottom and even if you did notice a difference it may be more subjective and not something that you would describe as being either better or worse.
Congratulations on your new mattress 
I’m looking forward to your comments and feedback once you receive it and have had the chance to sleep on it for a bit.
Phoenix