At the next general election all three parties will ask the British people to pay higher taxes in return for diminished services and benefits. They won't put it that bluntly of course, but that is what the promise of "austerity" means: pay more, get less.

So the issue isn't whether a period of real pain can be avoided. With an annual deficit of £175bn pushing the national debt towards £2.2tn, it clearly can't. The real issue for the next election is how the pain should be shared out. Who exactly will pay more and who will get less?

Of the many issues this question throws up, none is as politically potent as how much the "rich" should pay in tax. It is potent not just because it divides voter opinion, but also because it forces the parties to take a definite stand – to declare whose side they are on.

The contrast between the Conservative promise to raise the inheritance tax threshold to £1m and the Liberal Democrat pledge to introduce a new "mansion tax" on homes worth more than £1m underlines the point. Both parties will stand at the next election on a promise of "change". This policy difference – clear, concrete, impossible to fudge – challenges voters to decide what kind of change they really want.

At CentreForum we favour the Liberal Democrat approach for three reasons: it would bear down on inequality, boost social mobility and increase economic efficiency. The Conservative plan to cut inheritance tax would do the opposite.

First, the equality argument. Income inequality and, to a greater extent, wealth inequality have been rising for decades – a trend that the Labour government managed to slow, but not reverse. The fact that this trend is visible across the developed world suggests this is largely a result of global economic forces beyond any government's control. But government isn't entirely powerless. If it were to ask those who have benefited from globalisation to make a greater contribution, it could do more to support those who have been made relatively less prosperous and secure. The Liberal Democrat plan seeks to do precisely this. The proceeds of the mansion tax are earmarked, alongside those of other proposed tax rises, for the purpose of raising the personal income tax allowance to £10,000. This would benefit all taxpayers, but particularly the poorest, 4 million of whom would be taken out of the tax system altogether.

Second, the issue of social mobility. Those who are less concerned with equality might still agree that a just society is a socially mobile society – a place where people are able to climb the ladder of opportunity on the basis of talent and hard work. Yet most people who own or inherit million-pound houses, talented and hardworking as they may be, have accumulated much of their wealth by means largely or wholly unconnected to their skill or their labour. As Financial Times columnist Martin Wolf recently argued, property prices are in the main a reflection of location and amenities. This, he argues, makes property taxes desirable, as they "bear down not on effort, but on 'rent' – value over and above the costs of production". And as Sunday Times columnist Irwin Stelzer argues, inheritance taxes are desirable because they are paid only by "the lucky winners in the sperm lottery".

Which brings us to the economic arguments for increasing, rather than cutting, tax on property and inheritances. In exactly the same way as cutting tax on pay for extra work increases the incentive to work harder or longer, so cutting the tax on unearned wealth, be it inherited or passively accumulated from rising house prices, increases the incentive to work less or to stop working altogether. Which is why, with governments around the world looking to increase their revenues in the coming years, the OECD recently advised that "among taxes, corporate taxes are the most harmful for growth, followed by personal income taxes, then consumption taxes, with recurrent taxes on immovable property having the least impact." It is also why CentreForum argued earlier this year that as well as cutting spending, the next government should increase VAT to 20% and introduce a flat rate levy, over and above council tax, on the most expensive residential properties.

Time will tell whether it is the Conservatives or the Liberal Democrats who are in possession of the stronger political arguments. But if social justice and economic efficiency are the goals, there is little doubt which of them has the stronger policy arguments.