It depends ....
1) My personal opinion: Personally I love AD18 more than Cadence. It is a complete package - you have everything in one place from Schematic, libraries, pcb, output documents. It is also very intuitive and user friendly. Altium is the software what I have and use in my company (I also have OrCAD Professional, but I have not had the right project for it yet). HOWEVER, if I had a bigger team e.g. 3 and more HW engineers, I would seriously consider to go for Cadence, see the next point.
2) Price point of view: AD is getting more and more expensive (I think it is around 10 000 USD or even more, correct me if I am wrong) and Cadence can be interesting from price point of view. Based on the information I have, OrCAD Professional starts at $2K (for a standalone license) and includes advanced features. The OrCAD Professional has everything what you need for designing any board (you can use OrCAD Professional to design complex and advanced boards).
So, if your budget is limited, the 2k OrCAD Professional is perfect choice. Or, if your team is bigger, Cadence can make sense. You only buy a few expensive licenses and a lot of work can be done with cheap $443 license (I think that is current price for OrCAD Standard). Disadvantage of OrCAD is, that you may need some time to get used to it and they still need to improve some stuff. But I know they are working on it ...
Still, if you have for example 3 engineers, that would be like 6k vs 30k for buying the software ... big difference, and I would go for Cadence, even it is not as nice and easy to use as Altium (Altium has some cheaper options like CircuitStudio, but I would not go that way).
3) Big board design & Simulations: If your designs are based on big boards (e.g. server boards) or if you require simulations, definitely go for Cadence. This will cost you more money, but everything works together e.g. you can add info into schematic or into PCB and this info can be transferred and used in simulation. Cadence simulations are really good, Altium is not good in simulation. Also, a lot of big boards have reference designs in Cadence, this can save you a lot of time and money.
4) Your boards are used in mechanical complex design: consider Altium. I talked to some engineers who use Altium, because it helps them to work closely with mechanical guys.
5) Learning for future: If you are planning to be a hardware design engineer, learn both softwares
I am not sure if these points will help you, or they will confuse you even more
PS: Non of these companies is paying me, nor Altium nor Cadence, but I do have some free licenses from them.