After jumping more than 50% in three months, shares of low-cost upstart JetBlue Airlines ( JBLU) were downgraded on valuation concerns Monday. Meanwhile, Lehman Brothers had good things to say about Southwest Airlines ( LUV).

Bear Stearns analyst David Strine dropped JetBlue to peer perform from outperform, telling investors the company had reached his price target of $37, but stressing the downgrade had nothing to do with business fundamentals. Raymond James analyst James Parker downgraded JetBlue to outperform from strong buy on Friday afternoon because the stock was nearing his price target of $41.

In recent weeks, JetBlue shares have been pushing higher, because the company announced an ambitious plan to expand its fleet with a $3 billion purchase of 100 Embraer 190 jets.

But the company also owns an option to double that order, and that has given rise to concerns that it was expanding too fast and risking the same mistakes that helped kill People Express.

Both analysts stressed that JetBlue's business was on the right track. "It is important to note that this downgrade does not reflect a belief that the business model is less attractive because of the ERJ 190 order or that we perceive any missteps in the execution of the growth strategy," said Strine. "The change is solely due to valuation."

Likewise, Raymond James' Parker told investors, "Our rating downgrade reflects no deterioration in JetBlue's favorable growth outlook. Based on aircraft orders, we estimate JetBlue's capacity will grown about 38% compounded annually over the next four years and its EPS should grow approximately in line with capacity growth due to its low unit costs."

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