One other option: Do both.
You are free to argue that your IBS is
More than likely a functional disorder presumptive to GW exposure
At least as likely as not secondary to PTSD
Thus Service Connected Due to Presumptive and Secondary reasons
In my own claim I am arguing SC for several conditions that I have evidence could be linked to more than one condition. As an example, are my Migranes due to my
fibromyalgia
neck and shoulder conditions
PTSD
Allergies/Sinusitis
Directly connected as I was treated for headaches on active duty
It could be one of the above, or a combination of several conditions. In the end the arguments add up that they are SC because it is Most Likely it is one of the above and the RO can assign them to whatever he chooses.