That is tough...
Nokia Lumia 925 vs. Nokia Lumia 635 - GSMArena.com
I'd have to give the edge to the 925, even though the 635 is newer and has that quad core Snapdragon...
- Much better pixel density on the 925 - use an anti-glare screen protector for sunlight use (it helped me counteract the AMOLED sunlight washout)
- 925 has 1 GB RAM vs 512 MB on the 635 (lets you in for more App support)
- The 925 has a much better camera than 635 and isn't just the 3 extra Mega Pixels. 925 gets OIS + Zeiss Lens + dual LED flash + Pureview
- 925 gets Dolby headphone support
- 925 gets NFC
- 925 gets a larger battery
You're compromising thus:
- No SD slot as on the 635
- Lesser CPU and GPU
Unless storage and gaming top your list, 925 is better in the all-around. The 925 is a year older, but was considered high end on release. 635 is low-mid range all the way.