I have learned and used both Cadsoft Eagle and Cadence Orcad/Allegro over the last five years and applied them to major projects. Eagle is a very good program, best suited to a single users and applied to less complex designs (for example, a four layer board with hundreds of smt components on both sides including fine pitch ICs). The Cadence offering is now based on Orcad (aka Cadence Design) for schematic capture and Cadence Allegro for PCB layout. The Cadence applications have greater capability in several areas (for example, it is not stressed by a 16 layer board with thousands of smt parts, many fine pitch ICs and very dense signal routing, developed by a team of layout techs. Also Cadence can be applied to IC and other monolithic structure design too but I have no experience on this type of application). However, the Cadence programs require a much greater investment in training/learning to get up to speed. Unfortunately, this isn't just the result of the greater capabilities of the program. Perhaps half of the difficulty of learning Orcad/Allegro is (in my opinion) because the user documentation is very difficult to use. The documentation between Eagle and Cadence is a stark contrast. Where Eagle has a very good Tutorial and user Manual, that are easy to study and easy to reference, the Cadence documentation is hard to use and terrible for reference. I had no trouble learning Eagle on my own by going through the tutorial thoroughly and then reading every page of the Manual. In contrast, I was unable to use the Cadence documentation this way. To be honest, Orcad the schematic capture program is not so bad. It has been around a long time and so it is easy to find basic documentation written by third parties. Allegro, on the other hand, does not have this base of third party documentation available, and suffers badly from the Cadence supplied materials. In my case, it was necessary to hire a trainer to tutor me. I would guess that larger organizations would find it absolutely essential to hold classroom training sessions. In addition, I have found it frustrating and often hopeless trying to find answers to questions by referring to the Allegro documentation. The best way to find answers for Allegro, and often for Orcad as well, is to google the question in hopes of finding a user discussion dealing with it.
One thing that I like about Eagle is that it is designed for Windows from the ground up. When testing other pcb layout tools, I found many of the larger professional-type programs to be ported from older windows or non-windows versions. This makes them less attractive and sometimes less smoothly integrated and intuitive.
Eagle is a combination of two tightly coupled applications, a schematic tool and a pcb layout tool. The Cadence set is not as tightly integrated. Like most professional setups, the schematic capture (Orcad) runs somewhat independently and delivers a net list which must be manually imported into the layout tool (Allegro). This is a fairly common and normal method, but Eagle does this automatically (and thus is less prone to error). The automatic forward and reverse annotation in Eagle is a bit better as a result.
The Cadence system has a lot more functions and features that allow complete definition of a part within the tool using attributes and built-in spreadsheets. Linking to a company's part database is well within the Cadence capability but Eagle is weak in this area.
I especially like Allegro's feature where you can route a trace and the program will automatically move your trace around objects in accordance with your spacing design rules as you go along. Plus, you can move traces by automatically pushing them aside. Plus you can re-route a trace by redrawing a portion of it and the program will erase the old parts that you don't need any more. Nice.
The eagle schematic capture tool is a better graphical drawing tool. Orcad, by comparison, is quite crude. By this I mean, for example, if you want to draw your own schematic symbol, Orcad makes a poor drawing program, with very little choice in line widths, textures and patterns, grid resolutions, and Orcad often stumbles when you try to select a small object near another. As a graphics drawing tool, Orcad takes you back to 1990 in how it appears and feels. Eagle does not. This is a minor irritant, but bear in mind that the Cadence tools are more expensive than Eagle.
I'm not sure what it is about the design of Allegro, but it seems to me that related functions are not where you expect them to be in so many cases. Now, to be honest, sometimes this comes from things simply being different from the other pcb programs I've already learned. But I don't think that explains all of it. I find too often that a function that should appear on a menu accessed using a right click of the mouse button (based on context) is not there, and is only found in a less than intuitive location of one of the pull-down menus. There have been times when my tutor (with decades of experience with this tool) would say "hmm, I wonder where they have moved that function in this version?". So many times, I was unable to find an editing function under the Edit pull-down menu (for example). In contrast, Eagle does a pretty good job of this sort of thing.
To cost, you can lookup Eagle for yourself at the Cadsoft site, but it ranges from free for a hobby/student version to a couple of thousand dollars for a pro version. The Cadence pair will cost a lot more. How much more depends on what options you buy and how good your negotiating skills are, but $10,000 and up is the range.