I've been using Sinar P2's for 30 years, with 5x4 and 10x8 film, then later with a Linhoff sliding adaptor and a multishot Imacon 4040, then Leaf backs, and the Sinar has always been OK by me. Kind of a bit heavy, but they sit on studio stands so who cares.
I have the small Arca 6x9, whatever it's called, but I used it once and decided I preferred to stick to my habitual Sinar, the Arca is still intact in the boxes, a candidate for motor adaptation at some stage, when I find the time. The Arca bellows, you have to smell the quality leather once, and you are converted for life. But I am used to the asymmetrical axis of rotation of the Sinar standards, which I find is the quickest way to apply the Scheimflug principle. You need the smaller standards to make it work better for the small format of digital backs. So I recommend the P3, heavy as it is, but so sturdy.
But really, it's whatever works for you, it's just a tool after all.
For lenses and macro, I have both the Schneider Digitar 120 and the equivalent Rodenstock 120. I tried them side by side once or twice and I ended up using the Rodenstock, I don't remember why, because the Rodenstock is actually a little cheaper and I wanted the Schneider to be better, I think the Rodenstock has a slightly wide circle of coverage, and I think it was about a third faster using the same shutter and aperture, but either lens is superb. In terms of sharpness/definition, I can't choose between them. I also have the 150 and 180, but none are as sharp as the 120.
I am intrigued by the Actus DB, I may get one to tinker, it's so dinky:)
Edward