[General Questions and Answers] Planterz:
The Avant is the replacement for the Galaxy Light T399. I got the Light shortly after switching to T-Mobile from Verizon and needed something cheap but decent to tide me over until I could get a "real" phone (which, at that time, was going to be a OnePlus One, but not anymore). My feelings on the Light are very mixed. In some ways it performs very well. I get a screaming LTE signal at home (not as good at work, but far better than I did with VZW). I like the small size in the pocket but hate the small screen when using it. It does well on benchmarks, but in real use feels like it's just a bit underpowered, and has some severe lag at times.

The Avant seems to address most ofl the qualms I have with the Light. Bigger screen (but not a huge phone), more internal storage, FM radio. I always felt like the Light could use just a wee bit more RAM to run TouchWiz, and the Avant has that extra RAM. Almost everything about the Avant makes me wish it was available in March when I switched. Then again, I only paid $120 for my Light.

The rear camera on these 2 phones is the same, and I think the 5mp camera takes pretty decent pictures for what it is. Up close it takes great pictures. But farther away, it sufferers simply because of the lack of pixels. Perfectly useable for pics of friends or things to upload to Facebook, but obviously not great for scenic vistas or stuff in the distance. The 2mp front facing camera is obviously an upgrade over the .3mp of the Light, but I never cared about the FF camera.

I do find it curious that it not only supports T-Mobile's band 4 (which AT&T also uses), but also AT&T's bands 1, 2, and 5. Probably the same reason they released that unlocker app, but what that reason actually is, I have no idea.

Although I'm not in the market for this phone (I have a Nexus 4 now), I'm still very interested in it, at the least for recommending to others. On a payment plan, it's the best budget choice phone T-Mobile has. The Galaxy Light and the LG L90 can be had for $100 right now. The Light has LTE, but it's dinky. The L90 is bigger, but limited to HSPA+ 21.1Mb/s speeds. There's the Alcatel Onetouch Fierce, but you don't want that POS. Above that, you're looking at flagships or refurbs for a few hundred bucks more. The only thing in between that's new is the Avant or the Lumia 521 or 635, but who the hell wants a Windows Phone?

Off contract, the obvious alternate choices are the Moto G LTE from wherever or a Nexus 4 off eBay. The Nexus 4 is without a doubt the best spec'd hardware-wise, especially if you're willing to hack it for LTE and play around with custom ROMs and kernels, and can live with 8/16gb storage. Between the Avant and the Moto G LTE, it's a complete toss-up for me. The Moto G has the better screen, but the Avant has a removable battery. I don't consider the extra .5gb RAM an advantage for the Avant since TouchWiz needs it to compete with the minimal needs of the nearly-stock Android on the Moto G. The Avant can support a 64gb microSD card, whereas the Moto G can only take a 32gb (unless you format it to FAT32 and are OK with not having files over 4gb).

BTW, where are people seeing that this phone has a Snapdragon 400? I can't find actual confirmation of the processor anywhere. Both T-Mobile and Samsungs pages on this phone simply list it as a quad-core 1.2GHz. CPU-Z only lists it as quad core ARM Cortex-A7 at 300MHz - 1.19GHz (link).

Hopefully, especially being usable on AT&T's network, this'll become a popular low-mid end phone and will receive more development attention that the Galaxy Light has.