Both brands are aimed at young men, with both companies worried that their classic "diet" drinks are seen as too feminine.
The latest Pepsi marketing drive, called "Max your life", launched last month with an outdoor campaign followed by print ads, as well as a website tapping into the current craze for UGC, asking site visitors to submit videos and then vote on them.
It has now ramped up its activity, moving on screen with an ad featuring "hottie of the moment", Desperate Housewives' star, Eva Longoria. The ads feature a "Max meter", a gauge at the bottom of the screen with a can of Pepsi as the slider. The top of the gauge is Max and the bottom is zero - which just happens to be the name of a certain rival soft drink.
The ad shows two hitchhikers getting picked up by Longoria. She apologies for "not being able to take them all the way", before the car breaks down, edging the can up the meter.
After dolefully conceding they will all have to spend the night in the car together, Eva then suggests she helps the hitchhikers out of their wet clothes, sending the can soaring to "max". The ad is as crude as a Carry On movie, but does it work?
It has succeeded in getting people talking: since the start of the month Pepsi's "Buzz" rating has risen from -1 to +3. However, its general impression among its target male audience is unchanged on +4, while at the same time it has managed to alienate female customers - general impression of the Pepsi brand among women is down seven points from +10 to +3.
With the flat result among its key, male, demographic and a thumbs-down from potential female drinkers, it looks as though Pepsi might just have underestimated its customers' sophistication.
YouGov's BrandIndex is a daily measure of public perception of more than 1,100 consumer brands across 32 sectors, measured on a seven-point profile, with data delivered next-day.
YouGov interviews 2,000 people each weekday, more than half a million interviews per year.
This means you can spot trends as soon as they happen, not when it's too late. Respondents are drawn from an online panel of more than 130,000.
The score is the net rating: people are asked to identify the brands to which they have a positive response, and then those to which they have a negative response, to whatever is the prompt measure.
The net score is the positive minus the negative.
The seven measures that make the complete profile are below.
Each is taken independently - in any one survey, any individual respondent is asked about only one measure for the sector, not all seven. Therefore, none of the readings influence each other within the survey.
2. General impression
7. Corporate reputation
In addition, we supply an index score.
by Sundip Chahal