The ICM survey found 34% rated the Tory leader and 23% his Labour rival.
UKIP leader Nigel Farage was preferred by 8% and Lib Dem Nick Clegg 5% - although 21% answered "none of them".
Prof Roger Scully of Cardiff University said if Mr Miliband was behind Mr Cameron in such a "Labour bastion" as Wales, he could struggle elsewhere.
However, he added: "Margaret Thatcher won an election in 1979, despite being generally behind James Callaghan in opinion ratings," referring to the first of four consecutive Conservative victories over Labour.
People were asked to disregard their own party preferences when naming the leader they thought would make the best prime minister.
Support for further devolution of power to the assembly fell to 40%, from 49% in September.
Around one third of those questioned - 33% - said the assembly had sufficient powers, up by 7 percentage points.  Four percent said it should have fewer powers - up 2 points - with 13% wanting to scrap the assembly altogether - up 1.
Support for independence had doubled from 3% to 6%.
"There is a divide now between those who are reasonably happy with things as they are, and the somewhat larger group who would like things to go somewhat further, but not as far as independence," Prof Scully said.
Asked to pick which institution they thought was most likely to improve things for themselves and their family, 38% chose the assembly, 26% the Westminster Parliament, 20% their local council and 6% the European Union.
Prof Scully said this showed "a sort of broad, residual goodwill" towards the assembly.
As for Europe, nearly two-thirds - 63% - said they would prefer the UK to stay in the European Union, with 33% wanting to leave.
On university tuition fees subsidised by the Welsh government, nearly two-thirds of those polled backed support for students attending any UK university, while 37% said they should only be supported to study at Welsh universities.
BBC Wales' education correspondent Arwyn Jones said: "It's not hugely surprising that so many people in Wales are in favour of the Welsh government's tuition fee grant.
"It's the most generous offer to students of any part of the UK.
"The main opposition to the policy comes not from the public, but from the universities in Wales and opposition parties in the assembly.
"They say that over Â£90m is "lost" to them every time a student from Wales goes to study in England.
"Last week, a report by the independent Higher Education Policy Institute disagreed with that view.
"It pointed out that "arguing that the main problem is that institutions are losing money assumes it was theirs to lose in the first place and that they have first claim on the sums at stake. That is contestable.
"There is a review of the policy at the moment, but it won't report back for another year.
"One model being considered is that only students who choose to study in Wales would get the grant - our poll suggests that 37% of people in Wales would support that."
ICM interviewed 1,000 adults between 19 and 26 February 2015.

The contract - reportedly worth $10bn (Â£6bn) - was agreed on the sidelines of the World Economic Forum in the Swiss ski resort of Davos.
Ukraine is believed to have some of the largest shale gas reserves in Europe.
New technology means they are more easily recoverable, but opponents warn of a high risk of pollution.
The deal between Ukraine's state company Nadra Yuzivska and Shell was signed by the energy giant's CEO Peter Voser and Ukrainian Energy Minister Eduard Stavytsky on Thursday.
"We have witnessed a great event today. I believe we have become almost relatives," said Ukrainian President Viktor Yanukovych, who was present at the ceremony.
The 50-year production sharing deal is believed to be the biggest contract in Europe to extract natural gas trapped underground in shale rock.
However, experts say that production in Ukraine is several years away and much will depend on results from test wells.
Kiev is hoping that the deal would help it escape dependence on Russian natural gas.
Ukraine "might even go into surplus," Mr Stavytsky said.
Kiev's gas imports from Russia have long been a thorny issue.
In 2006 and 2009,  rows over the gas price led to Russia halting gas supplies to Ukraine during a bitter winter, causing temporary gas shortages in a number of EU countries.
More Welsh voters think David Cameron would be a better prime minister than Ed Miliband, according to a new poll for BBC Wales.
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Ukraine has signed a major shale gas deal with Royal Dutch Shell - a move seen as an attempt by Kiev to reduce its dependency on Russian gas imports.