If Israel were my one issue, then McCain would be the clear favorite. McCain will almost certainly continue Bush's Mid-East policy wrt Israel. If anything, lacking the Bush family ties to the Saudi royal family and the oil industry, he may even become much more insistent that Saudi Arabia and the rest of the Arab world shut down its support for Hamas' "civilian wing" (as well as support for its military wing).
With the Dems, it is absolutely no use pretending that a Democratic Administration is going to be more critical of Israel. The question is whether either Democratic candidate is likely to vary from the Democratic orthodoxy, which at this point can be neatly summed up by Bill Clinton's overall policy: lean on Israel to make concessions (but not too hard) while expressing sympathy for the Palestinians and pressing them (not too hard) to renounce violence.
My feeling is, no. Both will be equally mushy on the Middle East. Both will resist the efforts of Arab governments to make resolving the Israel/Palestinian question the necessary first step in resolving everything else in the Middle East (because Egypt can't resolve its own problems until the Palestinian Question is settled, dontcha know). And both will resist the most extreme sanctions pushed by Europe and the UN.
But the truth is that efforts to radically change the U.S. Israel policy founders on a critical point: there isn't a heck of a lot of leverage. If the U.S. were to attempt total divestment and economic sanctions against Israel, Israel would find a willing market for its goods in Russia, China, India and various Eatern European countries that don't give a crap about human rights. What makes a difference is whether the dynamics in the region are going to require substantive change by the parties on the ground. That's not something an American OPresident can effect.