Hi, the drugs you have mentioned are Histamine H1 Antagonist, belonging to Second Generation. These are newer anti histaminics which are devoid of sedation. That is the main advantage over the first generation anti histamines.
Both are mainly used in treatment of seasonal allergic rhinitis and treatment of chronic idiopathic urticaria. Headache and drowsiness are side effects seen with loratidine. Whereas headache and vomiting with fexofenadine. Both are quiet safe and any one can be used for which consult your local health care provider. Take care and regards.
Loratadine is considered a placebo by many doctors (and many patients that I have talked to). Zyrtec and Allegra are good choices. Since Allegra just came OTC, it will be much more expensive the Zyrtec generic. I take Zyrtec.
Take care and feel better.
God bless.