Flip side to the above reply is that doing the tunnel first may help avoid failing levels on the AFF and having to pay to repeat them.
If you do the general "tick the box on the bucket list" tunnel flying thing that people do, then it isn't as likely that it'll be suited as well for AFF, but if you make a point of letting the tunnel know that you are going to be doing your AFF soon, and are looking to get a good body position for that, they may be able to tweak things for you in your experience to suit the progression to AFF.
Maybe get the first level or two out the way, then get some tunnel time for the later levels where your instructor will let go of you... I wish I did, as that would have saved me 3 or 4 repeat levels.