You asked other posters which stays on their cameras most - which in the main has no relevance to you. You need to decide for yourself which you may use (or need, since your Helios comes closer to the 77 FL than the 31 does to the 15) the most.
I use the 77 more because it can be described as more of a general purpose lens (portraits, street and fireworks are some things I've used it for recently) than the 15 which is more specialised but a wonderful lens for night cityscapes, amongst a host of other WA uses.
I don't think you can compare the two and say one is better/sharper than the other because the 15 comes close to the best there is at or around that FL. The same can be said for the 77 in the 70 - 90 FL range.
Whats the idea behind those baffle's anyway? And cant they be removed?
Sure you can remove the baffle, but just be prepared to void your warranty in the process, though Canon got rid of it with the second iteration of the 85mm f/1.2L. The idea of the baffle on the original 85L is to suppress flare,they used that particular baffle on early versions of the EF 85mm f/1.2L - until users got ticked off with it, I still have one of the earlier versions of the 85L. The FA77 limited has a cylindrical internal baffle that holds the rear floating lens element, and the interior of that baffle is covered with what I take to be velvet - yet another quality touch from pentax. But you can still get straight edges, and corners appearing in bokeh because of the physical obstruction of the mirror box - something that mirrorless cameras do not suffer from quite so badly.
Geez~ If I were you, it wouldn't matter which one to get first.. Unless you're earning a living from it, which of coz would have helped you in determining which lens to get.
I honestly believe you should get the DA15 first... It's a totally different view and plaything.. Whereas for me, the 77 is just about rendition and bokeh and the hokey pokey. Know what I mean?
I would have bought the 15 first over the 77 when I was in the same situation as you a month back, it not for a mint A*85 that dropped from the sky.. LoL!!
Aren't the focal lengths (and speeds) so different that it is impossible to compare the optical quality in any meaningful way?
The DA15 is certainly capable of comparable resolution within the same field of view, and doesn't seem as prone to PF as the FA77.
Of course, one cannot make a direct comparison of the two lenses for the reasons that you stated. But as an owner of both, I can tell you that the FA77 delivers far better IQ. I'm not talking about resolution charts or lens flaws. I'm talking about the legendary rendering quality of the FA Limiteds, which I like to describe as smooth sharpness. Although I really enjoy using my DA15 for wide angle shooting and am quite pleased with the results, it is not in the same league as the FA77.