Its just my and my colleagues impression... AK100 sounds much better then Jr.
Its also has a much better form factor,
It has dual card slot
It has optical in
It has optical out.
It is sounds better...
I think AK do a very mediocre job in this device...
Form factor is subjective, but I think the AK Jr is the best form factor of the AK range. It is extremely thin and very very well made, and it is seriously "pocketable". AK100 form is a little too square and slightly more bulky to me.
As for the card slot, I am quite happy with the internal memory of 64GB (have not filled all) and a card slot with a 128GB microSD (which is completely empty for now). If they have included dual card slots into the AK Jr, it would have made a bulkier form, which is no good for me.
The buyers of AK Jr is probably not gonna use it with optical in since it is already capable of playing all types of lossless files, including DSD (convert to pcm). If they have included this, it will again make it bulkier and increase the price further.
Sound impression wise, no one can change your preference, but I had a good listening at the audio store (with AK100 and AK Jr) and I find them quite close, with AK Jr better in overall Sound Quality. Personal preference as well, and if you "see" a AK100 sounds better to you, you can always grab a AK100. AK made different models for different people with different needs/preference. Grab a DAP which is closest to your listening preference.
Sound preference is subjective and you don't have to go with the majority users who finds the AK Jr much more superior in sound quality. In fact, to my ears, it is much better than top tier DAPs that cost much more. This is after actual "hearing" of quite a few of them, not just seeing the form factor. In fact, I was deciding between a few other DAPs higher in price and was ignoring the "Junior" thinking that it is probably an entry level player which can't be any good. After trying out, I walked out of the store with a AK Jr, save the budget for a nice black leather case, and a highest speed 128GB microSD card. 1 week later, I end up selling off my DX90 too. It is that good to me, as it ticks all the box of my needs.
As for design of this device, I would say, kudos to AK for the great design and sound quality. Not many DAP comes close to this great design with good balance of form and features. It looks super nice in Blue, Red, and Black, but equally sexy in pure silver.