Caslon 3 is bolder.

540 has old style figures, small caps and swash variants. It's a more versatile choice.

Depending on your application, you might also look at Adobe Caslon, Big Caslon, Founder's Caslon and Lanston Caslon.

The digital version of Caslon 540 is better suited for display use--Big Caslon and the display cuts of Founder's Caslon are made for display. Adobe Caslon, the text cuts of Founder's Caslon, and Lanston Caslon are geared for text use.

Adobe Caslon has all the trimmings--small caps, swashes, ornaments--plus it's available in OpenType.

I haven't used Founder's or Lanston Caslon so I can't comment on them.

There was once an article titled "Which Caslon?" in Serif magazine. It compared: Adobe Caslon, Berthold Caslon Buch BQ, Big Caslon CC, Lanston Caslon 337, Caslon 471 BQ, Adobe Caslon 540, Bitstream Caslon 540 and ITC Caslon 224 BQ. I've only kept one page of the article, showing the title of the article in the eight faces. Of those, to me the Adobe Caslon (first one) seems to strike a good balance between authenticity and readability. Then there's HW Caslon, of course:
http://www.hwcaslon.com/

hhp

If you intend to use Caslon Oldstyle for text it is highly recommended you use Lanston's Caslon 337.

Lanston version ranked first ahead of Adobe in "Publish Magazine".

Gerald Lange I believe was unable to distinguish the difference between the digital version and the hot metal version lines inserted in "The Fount". Caslon 337 was made using the original Caslon punches for source.

Caslon 337 is a complete face, with oldstyle, lining, small capitals, full set of ligatures, italic, with swash and the long {s}.

Also it is complete with long and short descenders.

The Boston Globe used Caslon 337 for their anniversary edition.

http://lanstontype.com/CaslonFountEight.html

If you are reading impaired then you could take Hrant's advice into consideration.

Hrant"Adobe Caslon (first one) seems to strike a good balance between authenticity and readability."