To my knowledge there is no legal specialization in Tax Law. Attorneys who call themselves Tax Attorneys have had no specialized training in tax law whereas CPAs and Enrolled Agents have. So, if you want someone who can tell you the tax implications of various business entities, talk to a good CPA or an IRS Enrolled Agent.
There are Attorneys who are also CPAs. So they could give you tax advice and legal advice on business formation.
Some CPAs try to advise people on the best organization for their business, but they can only speak authoritatively from a taxation and accounting standpoint. There is more to figuring out the best organization for a business that just those two aspects and that’s why I recommend an attorney AND a CPA.