I've done it both ways.
Joint turned into a disaster. (just balancing EleGirl out
Two things that are probably the biggest influence to having separate accounts:
1) Do both of you work, and have similar/significant incomes?
2) Can you both agree on the plan, and be comfortable enough that it never becomes an issue?
As I understand it, you only have teenage step daughters, who should be off to college in a couple years, so I'm assuming your wife works and condition 1 is met. If so, and you don't expect to have babies anytime soon, then separate accounts can be a good way to not crimp each other fiscal decisions. But I also see form your post that you disagree on the plan, so no condition #2.
Here is what I would worry about
: Do you know your and your wife's spending styles? Are you two in agreement about how finances should be expect for the separate/joint account thing? .. and then you both walk the walk? If so, a joint account won't be a big deal.
Where it goes wrong is if one you is a spender and the other is not... especially if it involves taking on debt. Then one person can drag down the other.
It sounds like your marriage is new, so if you don't have a good feel about the two of you being on the same page.. proceed with caution.