This is an incredible price considering what you get. It's supposed to have a VRR screen too (which is huge).
In the next 1-2 years, if I continue to use my Deck often (hopefully), I may upgrade to ROG Ally. AMD's latest APU hardware is very impressive for its size and power consumption.
I bought a Steam Deck about a month ago. Bought the 64GB $400 version with the intention to upgrade it per the multitude of guides and suggestions online: Added:
SABRENT Rocket 2230 NVMe 4.0 1TB SSD ($150).
JSAUX 2-Pack Anti Glare Screen Protector 9H Hardness ($15)
SAMSUNG EVO microSDXC 512GB 130MB/s ($40)
Thermalright TFX 2g Thermal Paste 14.3W/m.k, 2Grams ($11) ... this dropped the CPU and GPU temps 3C to 5C for high demand games.
With the above mods, I'm very happy with the Deck. I play it more often than I thought. The Steam games are on the SSD and emulation games are on the SD (overkill space at this point). The interface works great and there's desktop mode to do "behind the scenes" tinkering like a typical Windows PC The great thing about the Deck is that it's been out a while and a few million units have been sold so there are tech savvy people that made software mods and enhancements (CryoUtilities, EmuDeck, Decky, etc). The Deck is a popular static platform like a console, so it's easy for these people to make these easy "enhancements" for it. I enjoy tweaking things so the Deck works great here. And playing NES, SNES, Genesis, N64, GC, Wii games on it are nearly flawlessly, which is awesome. I'm going to try a Switch game on it this weekend.
BUT ... the Deck's hardware is inferior to run modern demanding games. Even at 1280x800, there's no way to get 60fps in CyberPunk or Dead Space, for example. Even on lowest settings. Even RDR2 struggles (yes I read the many Deck tweak guides online). Other Deck players are satisfied playing these games at 30FPS in combinations of low to high settings. I don't do 30fps. 60fps was bare minimum. 120 is optimal. On the Deck, playing some favorites like Dying Light or Crysis Remastered at low-medium settings locked at 50hz play at 50fps nearly the whole time. I can live with this but don't prefer it. Valve should have sourced a VRR display.
Older favorites like Dirt 2, Dirt 3, Half Life 2, Trine, classic 90s games, etc always stay at 60-fps. Often I'll jump into classic favorites that run flawlessly on the Deck. I would also bring my gaming PC to friends' houses and play multiplayer games together for the night. Dead Island Remastered and Left 4 Dead 2 run flawlessly on the Deck so it's my new LAN party machine. MUCH easier to lug around lol. I tried a few other demanding LAN party favorites (COD, BF V). I hear COD runs great on it but haven't tried it. I'd want keyboard and mouse for it though.
My home gaming PC is an 119000K with a RTX 4090 connected to a 4k 120hz Samsung 9 Series QLED 75" that's GSYNC compatible, so I'm used to high refresh, max settings PC gaming. I find that, the smaller the screen, the less 60fps or even 50fps bothers me. Having a mobile PC gaming device is awesome. Not gonna lie.
Coming from that perspective, here's the synopsis:
Buy the Ally if you play newer games at medium-high settings (hopefully) and see yourself playing on the go or newer multiplayer games with friends at their place, or deck or patio.
Buy the Deck if you play older games, indie games, console emulation, don't mind 30fps for newer games, or play older multiplayer games with friends at their place, or deck or patio.
Don't buy either if you only play, or only intend to play, games at home in your room.