The intention in either case might be the same, but perhaps just saying one word may be a bit of laziness, yes. If we are really sincere in the wish, it doesn't hurt to give one full sentence to somebody, that is because Lovingkindness has a sense of generosity involved in it, a sense of giving. So if you find it hard to remember and enunciate all those words, it's okay to shorten it; but to shorten it to just "Metta" may indicate that we are not really sincere enough in really wanting to give a compassionate lovingkind wish to others.
Personally, I have a short form of a Compassion/Lovingkindness wish and I have a longer form. And we encourage you to do so, also, especially for those times when you don't have much time in your normal everyday life. And when you're trying to bring it into your everyday life - such as with Special Mindfulness Activities.
For example, yesterday's activity we gave you, was to give a Compassion/Lovingkindness wish before going into the meditation halls, dining rooms, etc. Especially in your normal life, if you pause too long to make the wish, then you may not make it into the elevator in time before the doors close. So it can be helpful to have a short form. For example, very short would be, "May they find peace of mind".