My first and only camera is a panasonic Lumix G5 with a panasonic 45-200mm. I wanted to have a first hand experience and decide if I want to switch to aps-c dslrs, but I never had an opportunity to use any PDAF system till now.I am having a hard time shooting birds, and I have no clue if it is the CDAF limitations, or just my skills aren't perfect yet. I don't know if a PDAF camera would make it a lot easier. I have read about 'birds in flight' shots and it appears almost everyone agrees it is difficult to shoot them with CDAF systems. But is it the same with perched birds (which would keep moving anyways, but I think they don't demand very quick AF as BiF?). If you have used both systems, how different are these systems in this regard?
I shoot both a Canon 50D and the micro four-thirds G3 and GX7. I use my 50D/EF 400mm f/5.6L USM combo for bird in flight shots. For me, the difference is chalk and cheese at the speed of reaction I have to have to get a BiF shot.
The G3 with my 45-200 OIS are perfectly capable of taking perched/walking bird shots, though, as you suspected.
Given that mirrorless cameras are now including on-sensor PDAF, a body like the Olympus EM-1 is probably somewhere between my G3 and dSLR performance, and that gap may close even more in future years with PDAF technology or lenses like the Oly 300/4. So, for now, I'd probably withhold judgement on whether dSLRs are always going to beat mirrorless. But I would say that at the moment, dSLRs have the edge for any type of fast-action shooting.
It depends on the camera, but in general, even a basic PDAF should be light years ahead of CDAF for any kind of action shot. The problem with CDAF is that it is a guess and check approach. The camera can't tell that it is in focus unless it tries going too far to one side and then too far to the other. For a still object, this works ok, but when the object is moving, it breaks down. The assumption is that the previous "best" value is still the best, but it might not be the case anymore since the object is moving.
With PDAF on the other hand, the camera can know for sure that it is in-focus at any given moment or know precisely how far out of focus it is. It is slightly trickier to use in that it can only focus on particular points within the image, however, it is far, far more reliable for getting focus on something that is moving. You simply place a focus point over the moving subject and let the camera find focus.
Technique still matters, but good technique with a PDAF is going to get better results than good technique with a CDAF for fast moving subjects in most cases.