The 920 is far cheaper, has double the memory, is damn near bullet proof as far as being dropped with no case on the phone at all and has built in wireless charging. If price, memory, durability or built in wireless charging with no case needed is important to you and you don't mind having last years model then get the 920.
The 925 is thinner and lighter with a sixth lens element and a slightly better display. So if your ok with needing a case for wireless charging at an extra fee of unknown amount at this point, willing to spend an extra $100 on the phone itself, ok with just 16gb of storage and want the updated version then get the 925.
In the end it just is a matter of your priorities on certain features / price.
As for me at this point I am torn between keeping my POS 8x or upgrading to the 925, I had every intent or upgrading two months ago when the phone was announced but now I question the price for features of the 925 and the super late release, not to mention we are now two months closer to the new nokia products that get released in November (just 4 months away) so I am thinking I will just wait it out for the new phones and GDR3.