The difference in display quality between the LCD Deck and the OLED deck is far larger than between the ROG Ally LCD and OLED deck. The old LCD on the Deck really was not a good panel and it had poor color accuracy and a low gamut making games look dull. The Ally has a high refresh rate 120hz panel with pretty good response times and better contrast than the original LCD deck. Sure it’s no OLED but the gap isn’t as large between them.
Now which you should get is really subjective. Both have their merits and downsides. The Deck OLED has much better battery life while the Ally has access to more games and other windows based software. Deck is set up for gaming first and foremost so you launch automatically into Steam Big Picture, Ally does not and launches into Asus’s Armory crate software. Ally can however be customized to launch into whatever program you want at boot up like Steam big picture.
Playing games via other launchers is much easier on Ally so if you own games from Ubisoft, EA, Epic, or other stores, you’ll have an easier time. Sometimes updates to these launchers will break games in Steam OS and it sucks.
Multiplayer games, or games with Anti cheat all just work on Ally while on Steam deck it can be a hit or miss whether it works. Some games just won’t launch on steamOS either, however thankfully we do have tools to help with this. ProtonDB is an excellent community run site that tells you whether a game works or not. You can pretty much ignore Steam’s playability rating as whether a game is “Great on Deck” or “Unsupported” really means nothing as many “unsupported” games work fine.
In my opinion no. Just get either a desktop in the same price range as that will vastly outperform the deck in performance while giving a better gaming experience on a larger screen. Deck should be bought mainly for handheld gaming, which is where it excels, just like the other PC handhelds.