Justgiving take more of the money for their own "costs" at 5% - virgin money giving are significantly better taking only 2%, and let you split the supported charities as you choose.
JG charge their fee on the total including gift aid, whereas the VM fee is based on the original donation with gift aid left untouched.
With just giving, if you donate £10 and claim gift aid, the charity receive £11.74
With virginmoney giving, that same £10 with gift aid means the charity receive £12.15
In the interests of full disclosure I work for virgin money (but not the charity side)