There's really no question in my mind - both. Which first? Doesn't matter, but I lean towards the PMP® as you probably want a job now, not 5 years from now. A recent Gartner report cites that about 1/3 of shops are using some form of Agile method, so it is gaining in use, and it will be a set of tools you will want in your kit. What has not happened yet is wide adoption outside software development. In that area, the Traditional Project Management (TPM) methodology still rules.
When it comes to qualification credentials for the exam, the ACP is lower (understandable given its less widespread use) but it's still not "light". You need 2000 hours of PM experience PLUS 1500 hours experience with Agile methods (non-specific, which is good). Total = 3500 hours. So if you're not already a PMP, you're looking at a couple years work to add up the hours. If you are a PMP, the 2000 hours PM experience are satisfied.
It's surprising, but Project Management is just recently starting to mature into a recognized profession by the masses. PM has been around for a long time, but when you reflect on the past 30-40 years, it's really only in the past 10 or so that you could tell your Mom you're a PM and she didn't go "What's that?" Agile still gets the question though (even from other PM's). Within "our little world" though, Agile is going to make some serious inroads I think pretty quickly. The idea of a 6 month project (of any type) being burdened with 3 months of PMO process on top (I just did a project like that) isn't going to last - it's just plain too inefficient. Having those Agile skills already at your disposal is going to put you ahead of the curve.
In practice though (initially), Agile is probably harder than TPM. It requires much more discipline and engagement from everyone. There are multiple methods and the method used on one project may not be exactly the same as on the next project of the same type. It's more important in Agile to keep a Team together, not throw them around from project to project willy-nilly.
The more you know and understand about all the "flavors" of Agile, the better you'll be at it. It is going to be a major paradigm shift for just about every company, and helping them transition is going to be part of your job. They (everyone!) have been trained for more than half a century in some form of TPM. Agile is a whole new way of thinking. Embrace change! Constant, consistent engagement of the customer with all the team members! Whew - that's a lot of emotional change for a manager trained in the '80's or '90's. They're going to need your help (and it's on the exam too).
Who in the world thought Project Management was easy?
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