Marmab,
You are in that good-news-bad-news situation where you have two sensible courses of action:
Why repeat ATG? Because you've been managing for a couple of years this way, you had some previous response to ATG, it's less risky than a transplant, and we know from the experiences of other patients that you could have an even better response this time. ATG might keep your counts stable for many years, possibly even indefinitely. Occasional transfusions could fill in the gaps. And if ATG doesn't work or doesn't last forever you'd probably still have the same two choices. Better treatment approaches might even become available in the meantime.
Why have a transplant? Because you are in good health and with a well-matched donor would be a good transplant candidate and could be cured of this disease. ATG might give you little or no boost. If you wait, you could be in worse health later, and have had a larger number of transfusions, and those factors would increase your transplant risk or even make you ineligible.
I've purposely stated some tradeoffs between these two choices. Is that the way you see it too?
I'm not a doctor, but in my opinion there are two factors that might force your decision:
It would tip the balance toward ATG if you are found not to have a good transplant match. Have you been HLA typed, have your siblings been tested, or has the bone marrow registry been searched?
If would tip the balance toward transplant if cytogenic abnormalities or excess blasts show up in your upcoming BMB. That's the situation my wife was in and it was the key factor that led us to choose the transplant route. That's the MDS side of the equation, taking over from the AA side.
You've educated yourself about AA and MDS so you know there's no magic treatment and you're weighing choices without knowing what will happen in either case. The doctors can look up statistics and make educated guesses but, as they say about investing in the stock market, your willingness to accept risk is a very personal decision. You've said you are reluctant to risk a transplant, and that's reasonable.
I know this is a tough decision. While it's beneficial to have both choices available to you, the pressure to make the call yourself is not to be envied.
I admire your doctor for giving you advice while respecting your opinion and accepting that it's your body and your choice. As long as you are on the AA/MDS border, I suggest that you continue to read what you can, including studying more details about the transplant process and considering who the donor would be. I think that delving into more details may lead you a clearer view of your own feelings about it, and you might end up agreeing with your doctor's recommendation or, conversely, knowing with more certainty that a transplant is not for you.