I know two systems so far: D&D 3.5e and GURPS 4e. What should I learn next?

[nothingforyou]

I'm generally looking for:

1) Simulationist games.
2) Hard realism games.
3) Non-combat games.

I don't expect many games to fit all three criteria, so I'm totally fine with multiple suggestions.

And a secondary question: Is there a migration from 3.5e to 5e for D&D, or will people continue playing 3.5e?

[Broken Twin]

Entirely opinion, but 5E plays pretty much like a simplified and better balanced 3.5. I'd advise at least giving it a look. Basic rules are free to browse online.

If you want something more similar to 3.5's level of crunch, I would look at Fantasy Craft. Personally, it's a very superior d20 system, and it handles both balance and out-of-combat options much better.

Outside of standard fantasy, Shadowrun is a crunchy fantasy/cyberpunk hybrid. Arguably fulfills your first request, partially fulfills the second, kinda works with the third.

If you want gritty realism/simulation, I'm told Riddle of Steal works great as a complex combat simulator. I haven't played it myself, so I can't say for sure.

... When you say non-combat, are you referring to games that have solid non-combat mechanics, or games that eschew combat entirely? Because there's not many that are both simulationist and combat-free, to the best of my knowledge.

[AstralFire]

I wouldn't really consider 3.5 or 5E to be particularly simulationist, realistic, or non-combat, honestly, so I am a little confused about your criteria.

Seconding Shadowrun, however.

[Delwugor]

If you are interested in Hard Science fiction then I'd recommend looking into Diaspora http://www.vsca.ca/Diaspora/

1. Simulationist - more simulation through mini-games, which captures the process instead of numbers.
2. Hard Realism - Nothing But Hard, though to be clear it does not go into gritty details.
3. Non-Combat - as it is based upon Fate it has mechanics for handling many situations outside of combat. Sometimes (such as social 'combat') it's clear cut rules and mechanics, while other times it's handled through base mechanics. As a nasty example, you won't find rules to handle a fire in a space station, but Fate provides several base means to handle it, such as Fate Fractals (Bronze Rule in Fate Core) so that the fire can actually "attack" and cause damage.

[Knaight]

I reccomend REIGN and Nemesis. REIGN in particular has a lot going on with non combat options (though it also very much has combat options). It also has a dice mechanic designed particularly to output more information about things, is generally pretty gritty, and is at least as realistic as D&D is. It's also fantasy. Nemesis is a modern action-horror game that uses the same core mechanic, clearly designed with simulation in mind. The other cool thing it does is looks at the psychological side of action-horror, simulating things like desensitization to violence, trauma induced phobias and triggers, etc.