In regard to which is better to control itching associated with allergies, there have been few studies that directly compare Allegra (fexofenadine) and Claritin. However, in the studies that have been completed, Allegra has been shown to be more effective suppressing histamine induced inflammation than Claritin. It has also been shown to work faster than Claritin.
One of the most common ways the effectiveness of antihistamines are tested is by evaluating how well they reduce inflammation from an induced "wheal and flare" reaction. A "wheal" is raised, red, itchy bump, which is surrounded by a reddened area, known as the “flare".
In one study, published in the Annals of Allergy, Asthma & Immunology, participants had a wheal and flare reaction induced by a skin-prick with histamine, which was then treated with either Claritin or Allegra. Allegra was shown the suppress the reaction faster than Claritin (2 hours for Allegra vs. 4 hours for Claritin) and produced greater flare suppression. The study concluded:
"In a skin test model of wheal-and-flare suppression, fexofenadine [Allegra] showed rapid distribution into the skin compartment with faster onset of action and greater potency vs loratadine [Claritin]."
Other studies, such as this one published in Clinical & Experimental Allergy and this one from the European Journal of Allergy and Clinical Immunology showed similar results.
Not all studies agree however. One study, published in Clinical Drug Investigation, reported that Claritin was superior to Allegra in treating seasonal allergy symptoms. However, it is important to note that this study used Allegra at a dose of 60mg twice a day, which is less than the adult recommended dose of 180mg once daily.
While the studies referenced above didn't specifically test Allegra and Claritin for treatment of itching or hives, they give us a good indication that Allegra may be the better choice as it works faster and is more effective in reducing histamine induced inflammation than Claritin. Nevertheless, either will provide effective relief.