Rotary or guillotine: which paper trimmer should I buy?

Across the cardmaking and scrapbooking communities we read, the rough consensus is that rotary trimmers (Fiskars, Carl) give the cleanest, most repeatable straight cuts on one or two sheets at a time and are preferred for crafting; guillotine trimmers (Swingline, X-ACTO) win when you need to cut a stack of paper, photos or cardstock quickly. Heavy crafters often own both.