I guess if you legitimately do not have the time to do the long mantra it is OK to do the short mantra. But if you can do the Long Mantra and you do the short one just to be lazy then I do not think much karma is purified since you are using a practice to create more imprints on being lazy...
hmmm...just some of my thoughts...
well short ones are not counted in the preliminary practices...go figure :p. the long one is called the 100 syllable Vajrasattva and most texts/pujas/practices would prescribe that instead of the short one. hint hint!!
Talking about preliminaries. For mandala offering, we do the 37 heaps mandala offering one round, followed by short mandala offerings, right? Or only the 37 heaps mandala offerings count?