You are asking for advice on whether pre-tax or Roth IRA is better, or split 50/50?
From the link above, "why" point 4:
4. Rule of thumb: traditional if current federal marginal rate is 25%; Roth if 10% or lower, or if MAGI is too high to deduct a traditional IRA; flip a coin otherwise.
You are basically trying to pay less in taxes by predicting now whether this year's marginal rate will be higher or lower than your marginal rate when you access your retirement funds. You have a good sense of this year's marginal rate. Your future rate is a bit murky... A lot can change over the next 40 years: tax rates could go up, or down. You might become a minimalist and spend little, staying in the 0% tax bracket, or buy a new boat every year and be in the top bracket.
If the answer isn't clear, then splitting 50/50 is a fine choice.
One point that may tilt your pre-tax/Roth ratios towards pre-tax: if you will eventually do the Mega Backdoor Roth as part of your savings, then you will get a lot of Roth funds from that. If your goal is to split your overall tax risk 50/50, then more pre-tax now makes sense.
Cheers!