The Keurig K40, K45, K50, and K55 are all the same basic brewer and collectively are one of the most popular models Keurig has ever produced.
When someone thinks of Keurig brewers, they're thinking of these models.
The K-Classic is Keurig's updated take on their flagship machine.
It doesn't have Brew Strength or Temp Control functionality and the 48 oz water reservoir isn't the largest you'll find, but not everyone needs those extras.
If you want the typical, hassle-free Keurig experience you've grown to love, stick with the K-Classic.