Both involve theft, except one is insured. You don't insure money that you give to an investor (Madoff), which is why it's disingenuous to compare Mt. Gox to an investment scam.
However, comparing them as a whole is disingenuous. MtGox had a legitimate business plan, but screwed up by letting their Bitcoins get stolen. Madoff planned to defraud his investors from the start.
Also, the term ponzi scheme get's used a lot in discussion about bitcoin, but many people do not know that it is a very specific type of scam. There have even been actual ponzi schemes involving bitcoins (not Mt. Gox). The people scammed in those were not insured because it was an investment, just as the people involved in the Maddoff scam were not insured. That is why it's disingenuous.
Insolvency wasn't the question.