They sound similar to me in overall signature / tonality, that same semi lean - slightly bright detailed presentation, both share an iBasso house sound.
The main differences are DX90 has better layering, slightly wider soundstage and is more revealing of detail (especially on firmware v2.2.0). DX90 packs more power hence the lower run times and simply sounds a touch more classy / effortless at everything DX50 does without any strain on your ears or higher volumes required to extract detail.
When I run Tralucent 1Plus2 with DX50 it sounds good, (firmware V1.6.0) but notably better on DX90 taking all above aspects further.
My suggestion is, if you're thinking to buy DX50 as the cheaper way out that's fine if your budget isn't flexible. In saying that, you'll probably only want DX90 anyway once you get DX50 always wondering, may as well go for the end result. If you're wanting to upgrade from DX50 then the benefits are there.
DX90 is a good unit, providing you let it settle in (if new) and keep realistic expectations (especially firmware v.2.2.0)
For example, I prefer my 1Plus2 with DX90 over Cayin N6, Pono Player, Hum Pervasion, Shozy Alien, DX50.