Well, I will address the accuracy requirement first. I have seven different brands of suppressor right now and another one in the queue. Every single one of them allows the rifle to shoot at least as well as it does without a can, and most shoot slightly better. I would say that it's easier to shoot accurately with a can because you aren't being effected by the concussive blast nearly as much as shooting without a can.
Point of impact shift is going to be an effect of having just about any can on a rifle. The effect can be fairly minimal though, and it should be consistent. I would say the best that I have is about a half inch straight down, and it's the same every time. You can set up your software to have an offset for shooting with the can and one without and then it's super easy. To be honest though, I never shoot without a can anymore. When I first started getting suppressors, I did do a lot of testing of this with several different systems and found most to be fairly repeatable. Some of them extremely repeatable.
Sound suppression, in general, is going to be a product of the volume of the can. The material that the can is made out of and it's thickness can change the tone quite a bit however. But on a meter, a can with 12.5cu.in. of volume will meter very close to another can with the same volume. Materials can definitely change the user experience though. In general, 17-4 stainless with a reasonable material thickness has a very mellow sound and will usually be considered quieter, even if the meter is saying it's the same. I have two fully titanium cans, and they are very light and repeatable, but the tone isn't as pleasant as the heavier 17-4 cans. Opinions vary on this, but not a lot of it is honest. There is strong brand loyalty among a lot of suppressor owners and sometimes, they aren't even honest with themselves.
I agree that a good system for attaching the can to the rifle makes things a lot more pleasant. Direct thread works ok, and a lot of people swear by that. I personally know that flat tapers don't stay tight very well, and would prefer something that will stay tight, as well as make it easier to put the can on and take it off and be repeatable every time. Just about every company uses some version of a tapered mount to accomplish this. I prefer the Griffin Armament system of taper mount, although I can't find any fault with the Thunder Beast or Rugged system either. I wasn't a big fan of the Silencer Co ASR system and I switched my Omega over to running the Griffin Plan "A", so that it would fit the same brakes that most of my other cans would. The reason I like the Griffin version is because they have a ton of options in thread pitch and brake or flash hider design, and they are nearly always in stock. The prices are better comparatively as well. All the brakes I have gotten from them having been high quality turnings and finished nicely with a qpq nitrid.
The choice of mounting system is actually a pretty big decision in picking a can, because it's inefficient and not cost effective to run multiple systems once you get more silencers. Luckily, a lot of manufacturers have settled on a standard thread of 1.375x24 for the breech end of their cans, so you can use your system of choice in a lot of different brands of cans these days.
As far as the muzzle aperture being 30cal or smaller, it's not much of a detriment to run the larger one. There will be a minimal, but measurable difference in a can made for a 6.5 and a 30, when shooting a 6.5CM. I feel that generally, it's a fair trade to be able to use that can on anything up to 300WinMag or sometimes even more powerful. 223 cans are definitely noticeably quieter on a 223, than a 30cal can though. Somewhere in the neighborhood of 3-4db. Sometimes that is made up for by the larger volume of the 30cal can. I would advise, if you're planning on only getting one, then get a 30.
One other feature that I would consider is whether you want a can that is user serviceable or not. Meaning, that the can comes apart and can easily be cleaned to return it to factory new levels of cleanliness. Even non-user serviceable cans may be cleaned, it's just going to take a bit longer, and it will probably never get as clean. At least that is what my bore scope has shown me. If you are considering shooting rimfire in this can, I would weigh this feature a lot more heavily. If rimfire isn't a consideration, then either choice is fine.
After saying all that, I think the biggest factor to consider when buying a can, is the quality of the company that made it. A lifetime warranty from a company that's going to be in business for 2 years, isn't worth much. This is a lifetime purchase, so pick a company that you feel will be there to help you if you have an issue. I've never had an issue that needed to be fixed, but I've had questions and the ability to get someone on the phone is a plus for me. I've had non-work acquaintances that made careless mistakes with their cans and needed the companies to help them make it right again. There are glowing reports out there from a bunch of the better known brands. I don't want to tell you what you should buy, but I will say that it's getting more difficult to pick a bad one. There are a few names to stay away from, and they are mostly companies that used to be very good, but got bought out and then customer service and innovation went south. In no particular order, take a look at the product offerings from SiCo, TBAC, Griffin Armament, Rugged, Dead Air and believe it or not YHM.
If you have more specific questions, don't hesitate to ask. Sorry for allowing this to get so long. It only took about 5 minutes to write. I gotta quit drinking coffee after lunch time!