No question, if you have an opportunity to get a YMCA certification, you should...one on one or otherwise. You will have an opportunity to get a CMAS card, because the YMCA is the ONLY certification that CMAS will recognize as up-to-standard.
Because of the CMAS card, you will have 2 cards to flash when asked to rent tanks, dive, etc. On that note, however, you will never run into a problem using your YMCA card. It is the standard and any sound dive operation will recognize it.
I don't feel that the instructor could make or break this deal for you under the YMCA. All YMCA instructors are held to a high standard.....
I took my courses through the Y, and watched my husband take a course through PADI...the difference is profound. Do not let the "other guys" convince you that your card will be unrecognized if you go with the Y, because they will try.
Happy, safe diving to you, whichever you choose.