On the face of it, the key issue is a technical one about financing future investment in London Transport. This is an important policy area, but the differences between rival mayoral candidates are less significant than they appear.
There is a broad measure of agreement that London's tube infrastructure needs cannot be financed simply by additional taxation, at least in the short term. There is equally wide recognition that the scale of the investment and the risks involved will not be attractive to private finance without a secure or generous rate of return. The arguments therefore are about how best to meet these requirements while retaining adequate public control over the network.
The main options are the private finance initiative or 'partial privatisation' proposal favoured by the government, and the issuing of bonds. The latter involves additional investment secured against a portion of future income streams. The government's option would see investment secured by surrendering control of existing assets, which only matters because it also involves foregoing future income streams. So the issues are largely doctrinal.
The bonds option is simple and has proved successful elsewhere, but has the disadvantage of requiring Treasury underwriting. The most likely outcome is some combination of the two, so why all the fuss?
The answer is that although the arguments about London Transport are important in themselves, they are also a proxy for a more significant debate about whether key decisions affecting London will in future be taken by the mayor or the government.
In part, we are witnessing another manifestation of the tensions inherent in constitutional reform. As in Wales, the spirit of devolution once uncorked cannot easily be put back into the bottle. The Welsh Assembly, granted only limited powers, has quickly agreed it wants more, and feels it cannot be effective otherwise. But this implies more power to take a different line from the government in Westminster, which does not give rise merely to political problems, but can create macroeconomic problems.
The Greater London Authority is not a regional assembly but will be more than just another tier of local government. The mayor will have a huge electoral mandate but few formal powers. The expectations the GLA will generate among Londoners will therefore exceed its ability to deliver. But voters are unlikely to be fobbed off by invitations to address their concerns to ministers.
So whoever is elected mayor will soon demand more powers, including revenue raising, and will have the support of most of the London electorate in this. So while the argument about financing the tube might look like a bruising encounter, it is in truth only the warm-up bout.