The tax gap from evasion is, give or take the odd billion or so, £70 billion at present. The total tax gap is about £120 billion.
Benefit fraud and official error combined cost £3.1 billion last year.
But apparently benefit cheating is times more important than tax abuse. How do I know? Because of this exchange of parliamentary questions and answers from Hansard:
Katy Clark: To ask the Chancellor of the Exchequer how much HM Revenue and Customs has spent on advertising for the purposes of preventing tax evasion in each of the last three years. 
Mr Gauke: HM Revenue and Customs spent £633,284 (excluding VAT) on advertising for the purposes of preventing tax evasion last year. There was no expenditure in the previous two years.
Katy Clark: To ask the Secretary of State for Work and Pensions how much his Department budgeted for advertising tackling benefit fraud in each of the last three financial years. 
Budgeted expenditure for advertising tackling benefit fraud
2007-08 £6.5 million
2008-09 £6.0 million
Note: Includes media costs, PR, production and research costs. It excludes VAT.
So over three years tackling tax evasion was worth just £633,000 but benefit fraud was worth £17.5 million.
So apparently, prima facie benefit fraud is 27.6 times more important than tax evasion over this period.
But weight the spend by the size of the problem and the ratio is even more spectacular. Then benefit fraud is 624 times more important than tax evasion.
Which is indicative of a spectacular error of judgement.