First - how would you assign ownership to her upon your death? I assume you'd be dead, being that upon your death, and unable to assign anything to anyone.
Second - It doesn't matter whether you assign ownership to your wife in the operating agreement (assuming you're talking about a succession clause to member's interest, talk to a lawyer), or will. In any case, tax treatment would be the same (consult with your lawyer and tax adviser, EA/CPA licensed in your state, to verify - I'm neither).
The fair market value (FMV) of your interest is added to your estate, which is then taxed as a whole. The executor of the estate will then assign the ownership to your wife (if it is in the will - then through probate).
As rightly pointed out in the comments, spouse 's (as recognized by the IRS) portion is exempt from the estate tax even if the estate as a whole is above the limits of the general exemption. Spouse may also re-use some of the unused exemption from your estate (this is new).
The estate tax is currently up to 40%, and with the exclusion amount being $5250000 for 2013 (adjusted yearly for inflation) I'm sure you can afford a professional advice if this is a concern for you.
Make sure to talk to an estate attorney and a tax adviser with experience in estate planning on what are the tax consequences of being dead.
There is a way you can transfer ownership to a trust to avoid "inheritance" or estate tax. You should consult a good CPA for professional tax advice. A good CPA is worth his weight in gold.