QUESTION 1: some have suggested that we open a traditional IRA and contribute some to that in order to lower our taxable income. However, we expect to be in a relatively low income bracket next year ($50-55k for a family of 3). Would you contribute to a traditional IRA, a Roth IRA, or both?
The main thing to consider with Roth vs. traditional is whether you expect your tax bracket to be higher, lower, or the same during retirement. If you expect it to go up, Roth contributions generally win. If you expect it to go down, traditional contributions generally win. If you expect it to be the same, either option is fine. You're currently in a pretty low tax bracket (15%). What about during retirement? Do you have some pre-tax retirement funds already that you would be withdrawing from? If you don't think these withdrawals will be enough to push yourself back up into the 15% bracket in an average year during retirement, you may want to make traditional contributions. If you expect your income to go way up before you retire, you may want to go with Roth contributions now while your income is low and make traditional contributions later when your income is higher.
Also consider the retirement savings contribution credit (also known as the "saver's credit"). This can give people a tax credit based on their adjusted gross income. It has a few different levels. If you're near the boundary for one of these levels, this can be a reason to make pre-tax contributions because they will lower your income and thus bump you up to the next higher tax credit level. You're not really close to the next level down (would need to get your adjusted gross income below $39.5k to do that), so don't worry about this for this year. Maybe in future years it will become a factor if you're able to save more, especially if your husband gets a job with an employer retirement plan available.
QUESTION 2: both my husband and I have Roth IRAs. Is it better to max out one account and put the balance into the other, or split the contributions equally? Are there tax advantages either way? Would the answer to the question be the same if we had a traditional IRA(s)? We are married filing jointly.
You should each put at least $2,000 in your IRAs to maximize your saver's credit. Anything beyond that, it doesn't change your taxes whether you put it in your account or his account or split it evenly. nereo's point about "just in case" situations is worth keeping in mind though.
BONUS QUESTION: we are both going to have (very) high deductible health plans for 2016. We each have an HSA, and the total balance in our HSAs is more than our out-of-pocket maximums. So, we have decided to save for retirement instead of putting more into our HSAs. Depending on our health care costs and work statuses at the end of 2016, our 2017 priorities might be different. Would you do the same? If not, why?
HSAs are great because you don't pay tax on the money going in
or out, as long as you have high enough medical expenses. You probably will have enough medical expenses to spend this money eventually, even if it doesn't happen this year. Be sure to get HSA accounts that allow you to invest your money in index funds to get the most long-term advantage out of these accounts.
I guess my overall advice for you this year would be the following:
1) Invest $2,000 in each of your IRAs to maximize your saver's credit. You'll probably get $400 back on your taxes for doing this, as long as your AGI remains below $61.5k. If your gross income ends up higher than this, make traditional contributions to bring it back down below this level. Otherwise pick Roth or traditional depending on what you expect your tax bracket to be in retirement compared to now.
2) Invest as much as possible in your HSA.
3) Once the HSA is maxed out, invest more money in the IRAs.