tl;dr - Plans frequently have differences in what is covered and how that can make an appreciable difference, but you need to do the math to decide.
For tax implications I would consult a tax adviser - usually medical premiums are deductible (it is unlikely this is worth it), but I do not know how it might affect state taxes where you live (if you have them).
Tenfour04 mentioned in the comments the bit about out of pocket maximum, but there are other similar considerations I thought I would mention.
Posted here for others, as well as a reminded for you to double check that there isn't a higher combined deductible, even though each person "has their own"
All the health insurance plans I have seen (WA state) have a per-person and per-family deductible. The per-family is usually some multiple of the per-person, the ones I have seen are either 2x or 3x. In the 2x case for a family of 3, you could wind up on the hook for at least a 3x difference (the 2x on the plan with kids and the 1x on the plan without). In general, if your family size is greater than the per-family deductible multiplier, you could wind up on the hook for extra in a split plan because of having to pay for one extra per-person deductible. You would need to math the difference (and of course factor in your personal health history as to if you think you will even hit the deductible).
Let's say you have two plans with a $250/$500 deductible and you use exactly the deductible. Unless your monthly premium is more than $20.83, you are out of pocket more.
If your family is unusually big, you become much more likely to hit the per-family deductible without hitting a per-person deductible for anyone, meaning that an extra plan just adds an extra deductible either way. There are other considerations, but this can be a noticeable one.
Note, the oop max usually follows a similar pattern (where there is a per-person and per-family that are related, and having more people makes hitting the limit much more likely).
Plans are not all the same. In addition to the deductible and oop max, you could have major differences in the plans for what is covered and how much.
Plans can differ in what they cover (some places allow for more variation than others), so one plan may cover needed medical benefits while another may not. A perfect example would be if one plan covers mental health and the other does not. For example, if you need regular chiropractic adjustments or counseling sessions, and your plan does not cover those but your spouses do, joining your spouses plan will almost certainly save you money (again, math it out to be sure).
Another consideration is the copay, and what services receive a copay, and if the deductible is waved in the presence of a copay or not. If you mostly get care in the form of office visits, this can be a big factor. The copay amounts can be different (e.g. $10/visit vs $25/visit), but sometimes the copay means the deductible is ignored for that visit. This is very plan specific, but that could wind up making a significant difference one way or the other.
Finally the covered amount also can make a big difference. If one plan covers 90% and the other 60% after deductible, this could swing the decision pretty decisively, especially in the event of a hospitalization or chronic condition. If you are healthy and usually don't even meet your deductible, this is less of a concern.
For a couple without children, or with only adult dependents, you can usually estimate your healthcare costs based on the previous years costs. In general, if you had back problems the year before you probably will, but if you didn't need to do more than the annual checkup, you likely won't need more than that again.
This changes drastically if you have small children. Children are wonderful in both their dedication to getting themselves killed, and their ability to contract every disease they have opportunity to. Just because a child did not use health benefits last year, does not mean they will not this year. While you should make your own assessments, I always assume that my children will use at least their full deductible.
There are a few simple rules of thumb that can be helpful:
- If your family is you and your spouse, you do not plan to change that before the end of the plan year, and the plans are identical, then take the two plans
- If your family routinely hits the out-of-pocket max, take the cheaper plan
Otherwise, you have to do the math for each plan with your specific circumstances to decide which route is better.