I will be back in the U.S. next month and will apply for a tourist visa. Will be in Thailand for a few weeks and then off to China for a couple of months. I don't want to use the tourist visa until I come back. Will the IO insist on using the TR visa if I don't list it on the arrival card?

Given the current visa exemption hassles I can only imagine they would wish for you to activate the visa.

They will probably insist on using the tourist visa on entry to the country.

You could get a 2 entry tourist visa to avoid the problem.

If they see it in the passport they will probably insist you use it.

As already mentioned a double entry would be the best option and solves the problem. Plus a single entry is only valid for 3 months from the date of issue and you must enter before the 'enter before' date on the visa. Based on your brief itinerary details you might be cutting it fine to enter before the 'enter before' date. A double entry is valid 6 months.