Whichever tour you choose, if any, make absolutely sure of the following:
Have the tour operator provide you a certified copy of their commercial vehicle insurance. If you are in an accident and get injured, crippled, or killed, you need to be certain the tour operator is carrying the proper insurance. Now, most of these guys, as Shane mentioned, will have you sign a form that acknowledges that you know you are partaking in an inherently dangerous tour; the form attempting to remove all responsibilities of the tour operator.
That is all well, and good, if a tornado picks you up or you get struck by lightning or soccer-ball sized hail; as you know this is a risk of storm chasing.
But an accident can occur when they are driving in clear weather, headed to a target area, or just going back to the nightly motel. You need to be sure their insurance will cover such an accident; as the driver could just as well be the cause--due to pure happenstance, because they made a bad driving decision, or because they were exhausted from the 12+ hours driving they often do in the course of a day's chase.
If the following are not in force with the tour operator's insurance, any claim will be denied by the company, once they investigate the circumstances of the accident and the overall use of the vehicle:
The insurance must be specific that the vehicle is being used in a commercial, for-hire enterprise.
The insurance must be specific in stating that the vehicle is being used in an inherently dangerous profession; and should really state that the vehicle is being used to intercept severe weather and observe tornadoes at close range.
The insurance must be clear in stating that the vehicle is crossing state lines and is 100% in effect in every state you will be traveling.
If the tour is using rental vehicles, you need to check the policy of the rental company as well; as most will not cover use of their vehicles for commercial enterprises, or cover use in doing things that are inherently dangerous.
Also, if the states you will be touring in require that any person involved in a for-hire enterprise needs to maintain a commercial driver's license, make sure the tour driver has a license that is recognized in all states involved, and is recognized for interstate commerce.
----------------------
Only a few, if not less, of these companies carry the proper insurance. The general line of thinking of the ones who do not carry the proper insurance goes like this: "We are taking chances all the time by chasing tornadoes. We are good drivers. The cost of commercial insurance for vehicles used in dangerous situations is so prohibitive, we would not make money if we bought it; so why not take more chances. We are either poor or are incorporated; therefore if someone or their family sues us for being crippled or killed in an accident, there will not be any real assets they can take from us to pay for their medical bills or incapacitation or funeral."
A person with this type of thought process is not the person you want to put your life in the hands of for two weeks running.
--------------------------------------
If the tour operator will not provide you a certified copy of their insurance BEFORE you sign up, call another company; as it is obvious they are irresponsible in their methods of operation.
There are a few tour operators that conduct their business in a serious, responsible manner. These are the guys to go with.