JSB .25 Heavy MKI vs MKII: Much difference?

I've been shooting the heavies for a couple years, in a couple guns. My previous .25's didn't like the MKI's. My current .25 shoots both equally well with MAYBE a slight edge toward the MKI's. But I'm shooting them a little faster, 890ish. That might make a difference. I would suggest you test them in your gun at your speed. 

my Vet does shoot the mk1's a little better also, prefers the aa25's over the jsb kings. 

If my memory serves me correctly, when the 33.95 MKII’s were released, the redesign was mainly to accommodate for the pellets to fit in more various PCP brands magazines. I believe some brands magazines had trouble with the head size or length of the 33.95 MKI’s This was documented somewhere on a popular YouTube airgun content creator’s review. But, I at this point have no recollection of who stated that. Maybe it was at a old Vegas ShotShow when one of the YouTube guys interviewed a representative from JSB. They perform exactly the same from my airguns, I use both..

From my RAW shooting them at about 890, there is minimal difference at 100 yards, with maybe a slight edge to the originals.

I have shot both equally well through my Impact. The mk2s are a tiny bit shorter...(910fps)

I shoot both out of my Impact (700mm with superior pellet liner) at 850ftps and can't really tell the difference at 50 yards. In my old smooth twist the Mark 2s shot better but I think that was related to the streamline magazines. 

I've never tried either of them in my Airforce Condor 24" barrel...has anyone had better luck with one over the other in their Airforce?

The original .25cal 34.95gr King Heavy’s are deadly accurate in two AF guns @ 950fps, a Condor and a Talon that we hunt with.

I push the MkII's at 940 ps out of my FX Wildcat MkIII with outstanding accuracy out to 70 yds. Beyond that they do suffer the same ills as all Diablo pellet designs. They are very "effective" on all targets of "reasonable" size at all those distances. They will still hold 3/4" at 70 yds.. I have installed a HUMA reg and set is a 145BAR. I do not clean the pellets at all. I do lube my pellets with Glen Larson's White label lube. The Sd's on that process averages in the high 6's - low 7's e.
Glen can be reached at https://forum.castbulletassoc.org/thread/395-white-label-lube-is-awesome/

The fastest my P35-25 will push either is about 760 fps. I've got them to 800 but getting a regulator pressure that the hammer spring to open is a challenge. I had it once but when the weather got warm the hammer spring was no longer adequate. So I just detuned to 760 fps. This is with an after market plenum. My gun is not as powerful as you guys are mentioning. It pretty strongly prefers the MKIIs. The MK1s are not terrible but are no better than a lot of other pellets. My Avenger is not as picky but prefers the 25s of the FX variety. JSBs shoot well too, however.

When I had an impact and now my Skout epoch I get very similar results with both. Result is excellent, guess I should mention that.

mkii shoot great out of my 2 .25s. leshiy 2 and impact m3

the mk2's did the best as a hunting pellet for me when they 1st came out from
my dream tact compact.
all is gone now.....