520 is lighter than the 525 and reduces the power taken by the drivetrain, giving you more power to the back wheel - also less inertial forces if I understand it correctly?
Don't know if you'd be hitting the power limit with a 520 chain on your bike though...
I used B&C express to source a Tsubaki chain. In hindsight their top-of-the-range Omega Gold is *too* big for the 748. Stick with the Renthal and matching sprockets. If it ain't broke...
http://www.bandcexpress.com/ Renthal Front 14t = £25.40
Renthal Rear 36t = £31.65
TSUBAKI SIGMA XRG X RING GOLD (94 link) = £75.73
Free normal delivery (order over £100), total price = £132.78 to you sir!
The 520 versions are same for front, £29.56 for rear sprocket.
You may want to go for a Regina or DID chain, but these are around 120 quid each.