blogging articles problem/question

hey all

i run a website where i have two main categories for my posts. Some are “main” posts and others are “articles”. I would like to have all the “main” posts – show up in full, and the “article” posts show the first 4-5 sentences, then a “…read more” link that will go to the full article. I can’t clutter my homepage with such long articles, and this is how i used to do it on my previous site. Is there a way this can be done ? i was thinking using the show categories plugin somehow ? im not sure. let me know what you think!

thanks all!