Sold my DX90 to fund the AK Jr.
Built quality is much much better. You need to hold both set to understand what I mean. SQ wise, it is up to personal preference. Source is always the smallest (or zero) differences (for the most demanding audiophile to be able to detect any differences) and thus as a normal user with average hearing ability, the AK Jr is at least on par if not better (in terms of my own sound preference) than my DX90.
I was holding on to both AK Jr and DX90 and compared both, before deciding to sell my DX90. I declare that I am no audiophile, as they both sound very good to my ears when I was comparing both side by side. Both have very good bass quality, and very detailed sound. Both can play almost all type of lossless and 24bit files. Given that they both sound as good to me with my IEM (I am using ASG2.5), the other factors become the key considerations. The slim factor as well as built quality of AK Jr is much better for the price (both sets are similarly priced in the stores in my country). It is built like a tank, and having both sets side by side, it is an easy choice.
After going through a long journey of different DAPs for the past few years, I will evaluate DAP in totality and not by audiophile reviews of dual dacs, sound stage, etc. The built quality, design of AK Jr is worth much more than many DAP with higher spec DAC/dual Dac, since they all sound good, but most don't look good at all, with some that don't even seem to be able to last for a few charging/recharging cycles. This is especially true for some DAPs that I have owned and sold, which had heat issues, loose screws, filmsy covers, faulty connection, etc. some were priced at more than double of AK Jr, but built quality is not even half of AK Jr.