This is EmuDevs, you'll find great support/answers here. As I mentioned in my post, I would recommend TC 4.3.4 overall. However, my experience with Arkcore and MaNGOS Cataclysm isn't that great, so you'd
have to download those and compare it. Although, I won't recommend getting MaNGOS since you'd have more problems generating/configuring in CMake AND compiling alone. It's pretty much between TC 4.3.4 and Arkcore 4.3.4. Arkcore is a forked TrinityCore that other people made separate from the TC repo. I don't know how experienced they are, nor do I know anything about their database (how great it is).
To be honest, you'll have a ton of startup errors using TC 4.3.4, a HUGE hassle updating your database structure to make it consistent with the most recent commit. I'm not sure if Arkcore always keep their database up-to-date without you pulling your hair out looking for the old sql files. That's the only problem with TC 3.3.5 and TC 4.3.4 databases, they don't know how to manage them correctly.
As I said, to be SURE and to find what you need, you'd have to setup both TC 4.3.4 and Arkcore to see which one is the best for you. Despite what I mentioned above, I'd still go with TC 4.3.4. Regardless, it isn't hard at all to fix Blizzlike content if it is missing/bugged.
Stability wise, I'm assuming they are great. I haven't had the chance to test how good the emulators do with a player base. As I've heard from people, it is great and sometimes it isn't all that great. Though, it will depends on how you handle everything; for example, clean database makes a difference, plus fixing issues with the database makes a difference too, fix console errors so it doesn't spam the console and cause potential lag and/or server crashes. There's a lot to know about stability, but you would just have to deal with the small stuff even if you don't think it matters.
I had a great server in 2009. The core was great, but it kept crashing for unknown reasons. Finally, all the crashes stopped when we fixed up issues and errors pertaining all to the database. Clean database is good. Clean code via server is also a must.
TrinityCore team did say awhile back when Cataclysm was around for awhile that they weren't going to focus on it much. Like I said above, they don't do much but small, minor updates and merge 3.3.5 updates into 4.3.4 branch. I don't get why they do that, wouldn't you think that could cause issues merging different patch source code into a completely different patch source code? Unless it is related to a certain system that works for both branches.
I could do a fresh Cataclysm repack on TrinityCore 4.3.4 (since I need to do that, been meaning to for awhile) and you can download it to see if you think it fits your needs. Sadly, I don't do Arkcore repacks since that is just a fork of TC with completely different updates. At least they've been working on Arkcore for awhile, so it
might have potential. Anyway, hope you found what you're looking for. One more thing, you can view my review on TrinityCore. I go in-depth about their Pros and Cons. Review:
http://reviews.emudevs.com/review.php?id=3