Should you buy in September, after selling in May?

It might seem a long time until September 13, but here at the beginning of May it's a key date. This year it brings Britain's classic horse race, the St Ledger. It's also the day when, according to the old saying, investors should put their money back into the stock market after selling in May.

There's been plenty of analysis on whether the old "sell in May" adage works and I don't intend to add to it. But because of the relatively flat, volatile and shaky start to the year for stock indices, it's a good point at which to gauge valuations. As ever there are opposing views, as evidenced by two of my regular CNBC guests.

Read MoreCNBC explains stock markets

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Bulls vs. bears

In the bull corner is Piers Curran from Amplify Trading. He correctly forecast the run-up in stocks to the end of last year and the flat trade for most of this.

In the bear corner is Doug Kass from Seabreeze Partners, who called the bottom of the market in 2009, citing it as a generational low.

Read MoreBubble talk catches fire among big-money pros

Curran says the three biggest risks to current prices are Ukraine, euro zone deflation, and the possibility of a weaker-than-expected Chinese economy. But he also provides some powerful antidotes, with potentially strong second-quarter U.S. growth and the latest U.S. employment report just the start.

In addition, there's the big pickup in mergers and acquisitions (M&A) activity, a sign that corporates are prepared to stop saving and start spending, as well as improving euro zone fundamentals.

Read MoreCNBC explains M&A

As a result, for the first time this year, Curran is risk-on. He says a bullish mood for equities over the summer could take the U.S. S&P 500 up through 1900 and the U.K. FTSE 100 to new all-time highs. These moves could be capped though when investors begin to focus on the prospect of rate hikes next year from both the Bank of England and the Federal Reserve.

Meanwhile, Kass compares today's changing market leadership with two other periods that presaged market corrections, 1972-1973 and 1999-2000.

He says bank stocks are typically a good market bellwether—and they have fallen both absolutely and on a relative basis this year. Bullish investor sentiment remains elevated and overly optimistic, and the buy-on-the-dip mentality continues to be the overriding investment mantra, with few expecting a serious decline. After dealing with a five-year period of ever-rising stock prices, bears have become an endangered species and Kass believes this provides almost no market cushion if things turn for the worse.

Also, Kass says the destruction of social media stocks over the last two months is a sign that investors and traders purchased positive price momentum in the sector without knowledge of fundamentals, company economics, and most importantly, without a sense of value. And worshiping at the altar of price works until it doesn't.

Two respected investors with the same start line but a very different finishing post.