Lots of free games, and most of the "trial" games are just a "buy the game to remove ads" trial; the game is otherwise fully functional. Even a number of Xbox Live games are free.
The FB app is pretty good, in my limited experience. I actually almost always just use the built-in FB integration and have no need for the app; just about the only things I use FB for these days are a great big address book + ability to see event invitations + ability to see posted photos, and the built-in integration handles all of that quite well. The app is pretty fast and fluid, with lots of features, though, as of the last update or two. There's always the browser in any case; touch.facebook.com works fine last I checked (or the full site, but it's not mobile-friendly).
There are many Youtube apps (plus again, you can just use the browser for HTML5 videos). Google has gotten into a pissing match with Microsoft over the official Youtube app though, so sometimes it's just blocked from working. There are alternative apps that I've never seen blocked (even though they work just like the MS app...), though, and there are apps for other video sites too,
Facebook messaging is built in to the "Messaging hub" of the OS, or accessible through the FB app; there are also dedicated messenger apps that can use it. Whatsapp is available. I don't know picsart at all, but there's some basic photo editing built into the OS, plus a crapload of apps for it.
Developer unlock is the ability to install unsigned apps (usually meaning apps in development, but also homebrew apps which aren't allowed in the store. It's the equivalent of the "Allow installing apps from untrusted sources" (or whatever) checkbox on Android. Interop-unlock is a hack that allows the installation of higher-privilege apps this way; without it, you can only sideload apps that have pretty limited capabilities.
All WP devices can be dev-unlocked; it's an official feature. If you want to sideload more than 3 apps at a time, though, you'll need to buy a developer account (~$20 USD/year). Interop-unlock is currently only available for Samsung WP8 devices, although Huawei phones *supposedly* can support custom ROMs (but this news is months old, and I haven't actually seen these ROMs myself) which would give the requisite unlocks too.
Do bear in mind: the phones you've mentioned are the lowest-end WP8 models, with the slowest CPUs, least RAM, least network technology support (no LTE, for example), and least internal storage (although I believe both support microSD, which can be used for photos/videos/music but not apps). Between the two, I would recommend the Lumia just because it will have Nokia's collection of apps.
Win8/Windows RT projects:
List of desktop apps for hacked RT devices
Native Access WebServer and Libraries
WP8 Interop Unlocks
XapHandler, Root Webserver, OEM Marketplace XAPs, Bookmarklets collection (Find On Page), Interop-unlock hacks.