The Avant seems to be T-Mobile's replacement for the Galaxy Light. The Light is currently on sale for $99 contract free (a hell of a price, btw), and the Avant has many of the same features, with many upgrades, so it's likely the Light is nearing it's EOL, with the Avant taking its place, and at the same price point the Light was when it was first introduced.
The Avant, to my eyes, fixes all the flaws that the Light had.
Most obviously, the bigger, higher res screen. The main thing "wrong" with the Light is that it's puny. The 4" screen made small text hard to read and made it difficult to type. The 4.5" screen of the Avant is a huge improvement, and the qHD resolution, even at the expanded size, increases the ppi from 233 to 245, so text isn't just bigger, it's clearer.
It runs 4.4.2 KitKat out of the box, while us Galaxy Light users are still stuck on 4.2.2, and although it was promised that it'll receive KK, it isn't looking too likely at this point.
16gb built-in storage vs 8gb built-in.
1.5gb RAM vs 1gb should make a huge difference with the laggy, bloaty TouchWiz.
FM radio.
2.0mp front facing camera vs 0.3mp, if you care about that sort of thing.
Bigger battery.
Oddly, it doesn't support T-Mo's upcoming band 12 LTE, but it does support 1, 2, and 5 (as well as 4, obviously), which are AT&T bands. So you're free to switch if you want, or buy one and unlock it (now that it's legal again).
------
So this phone isn't really meant to be an S5 "mini" (which'll be significantly more expensive), although one could consider it as such in many ways. Mostly, it fills a spot that's otherwise almost unoccupied in T-Mobile's line-up. There's the LG L90, which is similarly sized, but underspec'd and is only capable of HSPA+ @21.1Mb/s (but it can be had for $100 right now). There's the Nokia 635, which is a similar size, but has a lower res screen, far weaker specs, and runs WP, which will turn a lot of people off, but it has LTE. Anything better than the Avant, and you're looking at spending $400 for a LG G2 or Nexus 5 (better to get an N5 from Google, unless you need a payment plan), or $438 for an S4. There's a definite gap between the Avant and those phones, both in specs and price.
Of course, this doesn't take into account refurbs or unlocked BYODs. Obviously, to those don't only shop within the carrier stores, the main alternative to the Avant is the Moto G LTE. My personal choice in this price range would be a Sony Xperia SP 5306 off Amazon or a new 16GB Nexus 4 off eBay, both for $240. For $300-350, there's the Moto X or Nexus 5 from Google.
But it's obvious competitor is the Moto G. For me, it'd be a difficult choice. Moto G has the better screen, but I like hardware nav buttons. The Moto G has less RAM, but probably doesn't need the 1.5gb RAM the Avant has because it's running close-to-stock Android. The Avant has more built-in storage and can take a 64gb card, while the Moto G can only take a 32gb card. Hard choice, if you're not doing a payment plan.
I'd much rather have the Avant than my Galaxy Light, but I'm no longer in the market for such a phone, since I just got a Nexus 4.