Oppo 203D or 205D?

Have 105D aleady. Need a 4K player.
I say whichever you can afford or what is available if you want to buy now.

I own both; the 205 is better than the 203 in build quality and audio quality and justifies the price difference between the two (regular retail price not crazy mark-up price) IMO
if you need high quality audio then you need the 205, if you just want stunning 4k picture quality with ok sound then the 203 will work just fine.
As of right now there are still a few new 203 left for sale
I think all the sane priced 205 are gone, avoid the $3500 ones on eBay like the plague!
It looks like Oppo may do one final extra run on the 205, go to their sight and you can register by email for them to contact you about availability if they go ahead.
I have both and upgraded from stock 205 to the Modwright. Even through 4k HDMI bitstreamed Blu-Ray, the sense of space and subtle sonic detail is superior on the 205. 

The 203 is excellent and the 205 even as a pure 4k Player is absolutely sublime. 
The 205 has high quality playback.  It is ROON compliant so in addition to movies I use it to stream my digital collection using the analog balanced outs.  It makes quite a difference.    
New 205's are hard to find since Oppo announced they were getting out of the manufacturing business.