Real Estate Valuing

Would I be better served by using assessed value or real estate comps, to come up with "fair value" of the real estate in the will?

Depends on the purpose. If you're righting a will now and just want an idea of what different properties are worth comps is probably sufficient. If someone has died and you are obtaining values to determine how property will be distributed, or how valuable the estate is for tax reporting purposes you really need to obtain a formal independent written appraisal of each property. For commercial properties you might opt for a certified MAI appraisal. If its a personal residence, while a certified appraisal is recommended, many will opt for a comparative market analysis by a knowledgeable local broker.

Assessed value in most cases does not equal fair value. Even if it does (or if it is a percentage of fair value) for estate tax reporting you'll want to get an appraisal. Also, for estate and gift tax purposes you'll want to attach all valuation data to the tax return to begin the running (tolling) of the statute of limitations (the time during which the IRS can come after you).