Have you ever looked in the kitty's litter box and made a differential analysis? Me neither. And let me apologize right now for even asking you to think about such a thing. But that's about the best I can come up with in the way of a metaphor for distinguishing the qualitative difference between TV's new crop of prime-time big-money quiz shows, such as ABC's "Who Wants to Be a Millionaire" and the newest, NBC's "Twenty-One." I suspect that even Teddy Nadler, Robert Strom and Elfrida von Nardroff would be appalled. (They were the biggest winners during TV's last, ignominious go-round with prime-time big-money game shows. Nadler won $252,000 on "The $64,000 Challenge," Strom won $224,000 on "The $64,000 Question" and von Nardroff won $220,500 on "Twenty-One" — all back in the 1950s before the Feds found out the shows were rigged.) Since I'm forced to look in the litter box and make a judgment, I have to say that "Millionaire" has it all over "Twenty-One." But that, obviously, isn't saying much. However, to its credit, "Millionaire" is just a tad slicker, with its flashing lights, neon set, and frenetic music. And, heaven help us for even noticing the difference, but "Millionaire's" Regis Philbin rates a bit higher on the unctuous host scale than does "Twenty-One's" Maury Povich. But that's kind of like saying your garbage can stinks more than mine does. Who wants to cozy up to either one? "Twenty-One," which debuted earlier this month, is based loosely on the card game blackjack. Two contestants compete with each other to earn 21 points and win the game. As each round begins, the contestants — who are in individual "isolation booths" — are told what the category is. Each in turn attempts to answer a question worth from one to 11 points, the questions being more difficult as the point value increases. Questioning continues until one player reaches 21. If a contestant misses three questions, he's automatically eliminated. As on "Millionaire," "Twenty-One" doesn't seem to test one's depth of knowledge but one's range. For example, on the show's debut, the first question — a seven pointer — was "Who plays Ray's mother on 'Everybody Loves Raymond?'" Another seven-pointer was "What product uses the slogan 'When it rains, it pours'?" And just to make certain it's not too hard, the questions are all multiple choice. In no time at all, the first winner (Jenny Fernandez from Alexandria) accumulated $160,000. Then she was eliminated when she didn't know that Oddjob was the character who used a hat as a weapon in "Goldfinger." Sad and simpleminded though they may be, prime-time quiz shows are multiplying across the networks because they are ratings winners and cheap to produce. Stay tuned. Next, they'll be retooling "Dotto." Then we'll know it's time once again to empty the litter