The payoff for a natural (a two-card 21) on single-deck games was reduced to 6-5, from the traditional 3-2, 10 or so years ago and has become the rule at most tables ever since, especially single-deck games with low minimums on the Las Vegas Strip.
All things being equal, it's much better to play at a blackjack table that pays 3-2 than 6-5, same as it's better to play a 9/6 Jacks or Better video poker machine over an 8/5 JoB or a single-zero roulette wheel over a double-zero.
To get technical, the short-pay on blackjacks amounts to a -1.4% penalty on what's already a lousy game (since modern single-deck games are usually dealer hits soft 17, no double after splits, and sometimes double 10 only), putting the game in the neighborhood of -1.8% off the top.
But what most players don't know is that it's actually worse than that. The casino pays every $5 of a bet at 6-5, but then any portion of the bet remaining that's less than a $5 increment gets paid at even money. Hence, a $7.50 bet that gets a natural is paid only $8.50 (instead of the $9 proper payoff), which works out to 5.67-5 blackjack!
It's true that many players resent the 6-5 payout, but many others simply don't understand the difference. Indeed, Bally's -- one of the first casinos to deal it -- advertised on its marquee that the game was being dealt "by popular demand."
As always, there can be exceptions, but the rule of thumb is to avoid 6-5 blackjack in favor of a game dealing any number of decks, even 6 or 8, that pays naturals at 3-2.