yes a marriage alliance will usually last much longer and is more reliable than a simple alliance...

Furthermore, IIRC, Marriage alliances have relations remain to the good for more easily than with a simple alliance...

usually, a marriage alliance is useful to keep one front secure while fighting on another...

Say, you are the French, if you want to get rid of the English you should sek a marriage alliance with either the HRE or Milan as they are most likely to attack you...