It's a fairly artificial and arbitrary distinction. There's a lot to be said for seeing it as a matter of two varieties of the same sickness.

The two prejudices have historically tended to go together, and they still do frequently, for example among the followers of Le Penn.

In fact back at the beginning of the last century it was common to find Jews and Arabs being bracketted together in anti-semitic writings, and seen as two aspects of the same enemy.

The same was true in Spain earlier, where Jews and Arabs were treated the same way when it came to persecution and expulsion.

Or consider Othello:

And say besides, that in Aleppo once,                     
Where a malignant and a turban'd Turk
Beat a Venetian and traduced the state,                     
I took by the throat, the circumcised dog,
And smote him, thus.