I am NOT a financial advisor, but I will attempt to give you a few things to consider. First, with regard to EITC, you need to know that the EITC calculations consider BOTH "AGI" and "Earned Income". EITC is calculated using both AGI and Earned Income and you get the lower benefit. When you contribute via your 401k, you lower both the AGI and the Earned Income. However, note that you must have Earned Income in order to be able to contribute to an IRA. Therefore, while the IRA contribution can lower your AGI (due to the IRA adjustment), the IRA contribution does not affect the Earned Income number. I suspect that is the reason you see no affect on EITC. You can consider increasing the 401k contribution to lower both Earned Income and AGI in the future.
Now with regard to choosing deductible or non-deductible for IRA, please be careful. Your comments make me nervous. In any case, please keep good records with accurate information and your Form 8606s. Most of people, whom I know, usually choose deductible contributions for Traditional IRAs in order to take advantage of lowering their out-of-pocket costs for the contributions. If you make a deductible contributions, then that should lower your AGI and it should lower your tax burden. Likewise, the people, whom I know, will typically use a Roth IRA when they can afford to fund their IRA savings with after-tax money. (There are additional advantages for Roth IRAs that are beyond the scope of this short response. Please study the benefits of the Roth IRA.) If you are not seeing any benefit from making a deductible IRA contribution, then please investigate to understand what is happening. If you make non-deductible contributions, that can lower future taxes for withdrawals (again good records and Form 8606 are essential), but please look into the Roth IRA as an alternative if that is your situation.
Also, be aware that there are limits for Traditional IRA contributions when you have a retirement plan at work. See: https://www.irs.gov/retirement-plans/plan-participant-employee/2018-ira-contribution-and-deduction-l...
