Actually Audio engr would be the best person to answer this.
Both these are 2 way vented box speakers.
The 685 is at Rs 50513/- all inclusive in mumbai. UK price is much lower. This is a popular speaker and is reasonably good. Its the larger bookshelf from the B&W 600 series which is now made in china. Specs/materials can be seen on the BW Site. Same old Kevlar bass/mid driver with a fixed bullet. Simpler crossover and an aluminium tube loaded tweeter.
Tonally good, with smoother HF as compared to the speakers it replaced i.e 601/602 and overall a good speaker at the 30/35K price point which is where it should have been!
805s is the only bookshelf from the 800 series and does not feature the diamond dome tweeter rather relies on the simple Nautilius alluminium dome tweeter. Heavily Braced (matrix) with a similar Kevlar mid/bass driver. The tweeter is mounted on TOP of the speaker fpr obvious advantages. Sensitivity is 88db and price is 198000/- all inclusive.
So its about 4X the price. Is it worth that much, I cant say. I have it for about 2 years and I paid about 1.25L for it. It continues to amaze me. Warm mids/adequate bass(quality & quantity) and super highs. Metal tweeter can sound bit harsh if poor amps are used so one has to keep a fair budget for amplification and source.
B&W insists that the speaker can be driven by low powered amps (50W SS) to satisfy but I totally disagree. The best results I have had are with Quality/ high powered SS amps.
For either speaker u will need a solid stand which u can get made as the ones B&W sells are pricey and fairly average in build.