It would be a tough decision as to which I liked the best. Below is my 627 Pro, SKU #178014 & MSRP $969, which I bought new 3+ years ago, and my 2 5/8" PC627 UDR, #178133 & $1,049. which I bought new 1+ yr ago. Both have aftermarket grips - and the 627 Pro has a HiViz sight, too. Both come pre-dished for moonclips. As everything I own can be used for I keep a number of moonclips (50/$50 from Ranch Products) loaded with Remington R38S12 +P 158gr LHPSWC. The 5starfirearms.com speedloader/loading base works great, too - with any .38's or .357M's. The moonclips are a bit mor particular!
If I had to get rid of one of the above, I'd say goodbye to the UDR. Of course, I cheat - I bought a second one last October - so I'd still have one! I actually bought the second one for a HD gun - it even got a Lobo Leather OWB holster. Super revolver.
In use, they are very nearly equivalent - despite the greater sight radius of the 4"-er, I can take out the SPC targets at 12yd just as reliably with the UDR. It's lighter, too - by a whole 3.6 oz. I liked the similarly sighted 625JM's grips - got a JM grip for the 627 Pro. BTW, the 627 Pro - like the 625JM - has a spring-loaded front sight mount for fast tool-less sight changes. The UDR has a bright orange ramp - on a dovetail. It's quite visible in high or low level light. Good thing... other choices will be few - and custom made, it seems. Both came with trigger improvements, the UDR's being a tad better - and eased ejector stars for faster reloading.
Ah, but you wanted a 686P comparison. My only 686P is my 5" half lug 'Stocking Dealer's ExclusI bought new 12/04. It's the one with cobwebs on it - in the back of my safe. I enjoyed it over my 6" 66, but neither had a chance once the 627 Pro arrived. They are about the same weight. The Pro and the UDR need different wood grips, the Pro having come with OEM Goodyears and the UDR with it's poorly fitting Eagle boots. Both can take the same exact grips, if need be - they are RB N-frames. I have tried other grips - Ahrends retro targets& square-conversion - all in cocobolo - and prefer my 627s as shown. Hope this helps.
Stainz