The 5900 is a replacement for the 4200.
The 7900 is a replacement for the 5200.

The two cameras are identical except for the megapixels.

According to Amazon.com:
The 5900 will be shipping on April 10.
The 7900 will be shipping on May 1.

Estimated street price:
5900 - $350
7900 - $450

"Are they worth it?" -
Well, it seems we get more for our money each year.
5 megapixels for $350 doesn't sound bad.

"Which would you buy?" -
That would depend upon the need for more megapixels.
Do you have a need for 7 megapixels?

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