Primerica's primary value proposition is that switching from whole or universal life to term life, and investing the difference is a good idea for most people. However, there are a number of other important factors to consider when purchasing life insurance, and I would also be wary of anyone claiming that one product will be the "best" for you under all circumstances.
Without getting into a much larger discussion on how to pick insurance companies or products, here are a few things that concern me about Primerica:
They have a "captive" sales force, meaning their agents sell only Primerica products. This means that they are not shopping around for the best deal for you. Given how much prices on term life have changed in recent years, I would highly recommend taking the time to get alternate quotes online or from an independent broker who will shop around for you.
Their staff are primarily part-time employees. I am not saying they are incompetent or don't care, just that you are more likely to be working with someone for whom insurance is not their primary line of work.
If you have substantial reason to believe that you may someday need whole life, their products may not suit you well. Primerica does not offer whole life as far as I am aware, which also means that you cannot convert your term life policy through them to whole life should you need to do so. For example, if you experience an accident, are disabled, or have a significant change in your health status in the future and do not have access to a group life policy, you may be unable to renew your individual policy.
Above Average Returns?
I am also highly skeptical about this claim. The only possible context in which I could find this valid would be if they mean that your returns on average will be better if you invest in the stock market directly as compared to the returns you would get from the "cash value" portion of a life insurance product such as universal life, as those types of products generally have very high fees. Can you clarify if this is the claim that was made, or if they are promising returns above those of the general stock market?
If it is the latter, run! Only a handful of superstar investors (think Warren Buffet, Peter Lynch, and Bill Gross) have ever consistently outperformed the stock market as a whole, and typically only for a limited period of time.
In either case, I would have the same concerns here as stated in reasons #1 and #2 above. Even more so than with insurance, if you need investment advice, I'd recommend working with someone who is fully dedicated to that type of work, such as a fee-only financial planner (http://www.napfa.org/ is a good place to find one). Once you know how you want to invest, I would again recommend shopping around for a reputable but inexpensive broker and compare their fees with Primerica's.
Kudos on having a healthy level of skepticism and listening to your gut. Also, remember that if you are not interested in their offer, you don't have to prove them wrong - you can simply say "no thank you."
Best of luck!