As Thaiman said.
Beresford TC-7520 is around £180 including VAT and UK postage. (1 optical + 1 USB + 2x coax digital inputs).
Beresford TC-7510 just over £140 including VAT and UK postage. (2 optical + 2x coax digital inputs).
The TC-7520 is somewhat better sound quality. Both models have built in headphone amps and either fixed or variable outputs.
The TC-7520 can have it's opamps user upgraded easily in a few minutes with slot in components costing £10 or less from ebay with no soldering or electronics skills required.
The TC-7510 cannot be user upgraded without good soldering skills and a knowledge of what you are doing.
More info here.
If you only need 1 optical input and like better quality and you are the sociable type - who wants their mates/missus/GF with (USB only) laptops to be able to use the same DAC - then the TC-7520 is the better match.
If you don't need USB and prefer 2 optical inputs (for a second device like a Freeview/Sky/Virgin box or PS3 or whatever) then - despite the slight quality difference - the TC-7510 would suit better and save you £40
Rega P2 • Ortofon 2M Blue • Rega Fono Mini • Naim Nait 5i • Naim CD5i • Naim NAT 05 FM tuner • Rega R3 loudspeakers • Naim NAC-A5 speaker cable • Beresford TC7520 • Panasonic DMR-EX78 DVD/HDD • Chord Chrysalis, Crimson & Naim interconnects