Your answer is that you really should attempt to enter them both. If you look carefully at your W-2 data entry screen in TurboTax, you should see multiple "drop-down" menus in the software where you can select multiple letter codes for Box 12 (a, b, c, d). If you use your mouse or pointer, you should be able to chose the DD letter code for 12a, and the W code for 12b.
Please see the screen-capture images attached to the bottom of this answer for a visual guide (simply click to open the pictures).
That said, if for some reason you are only able to enter one single value for Box 12a, then make sure you pick the W letter code and the associated number. The reason we say that is the letter code DD is simply a reporting mechanism for the Affordable Care Act (also known as Obamacare) that tells the IRS how much your employer spent on health insurance coverage for you during the year. It doesn't actually affect your tax return in any way.
On the other hand, code W is for employer contributions to a health savings account; and this has actual tax consequences in that it can raise or lower your overall tax refund. Thus, if you must pick one code over the other for Box 12, choose W.
But, please first try to enter both Box 12a and 12b codes, using the example in the images, if at all you can.
Thank you for asking this question.
