I have posted this before, but it is appropriate again here, I think. I made a PDF of several shots I took in Utah a few years ago, comparing 12mm (P12-35 lens) vs 9mm (Oly 9-18 lens) from exactly the same vantage point. There is a BIG difference. Of course, 8mm would be an even bigger difference. See it here:
I am a little puzzled by the lack of mention of the PL 8-18 f2.8-4 (which I have). Compared to the 8-25, it is smaller, lighter (411g vs 315g), faster (obviously), and somewhat cheaper, I think (without a sale on the 8-25). Yes, the 8-25 has a better far-end focal range, but much of the discussion here is about the ultra-wide aspect of the 2 lenses, and the 8-18 is better at that Focal range (faster; and the size).
If I didn't have the 8-18 I think I would not be tempted much by the 8-25 (over the 8-18) because that's not my primary use for an ultra-wide lens like this. I CAN see the the advantage of say the 12-100 over other general-purpose lenses like the PL12-60 and the O12-45 (if you ignore the size), because it excels at the focal lengths you might likely USE it for.
Note the 8-18 lists for the same price as the 8-25 (MSRP on both is $1099USD), there's just often a sale on the 8-18 (it's $200USD off right now).
There's a few advantages for the 8-25
1. All the Oly toys are available. You don't get locked out of some automation (Focus stacking, full ProCapture support, probably a couple other things)
2. Likely better sealing (IPX1 rated, the new front element coating that repels water, plus rear seal matches to the Oly Body's design)
3. Filter compatibility (72mm is one of the two common filter sizes for PRO lenses, as well as the Oly 100-400, 8-18 uses 67mm filters which means an adapter is needed if your other lenses use something else, conversely if pairing with a 12-40, the adapter needed for the 12-40 is smaller when paired with the 8-18 because the 12-40 is 62mm. Note the 12-60 is also 62mm and the 50-200 is 67mm so the 8-18 pairs better with the other PL lenses)
4. MF clutch - much quicker AF/MF switching and you can preset focus distance (I was surprised by how much I use this on the 12-40, I've owned traditional clutched lenses in the past and hated the feature because of how poorly it worked in my experience, but the combination of clutch and focus by wire makes it much more flexible).
My take on it is largely that if I already owned the 8-18, I probably wouldn't buy an 8-25 unless I was looking for that 2 lens hiking kit, but if buying a working kit, I still would prefer to match lens lines to the body lines, so I'd get the 8-18 for a G9 or GX9, but the 8-25 for an E-M1.