The best trimmer of all is the guillotine (bypass) type. There are several on the market. I have a couple of older ones with wooden bases (purchased years ago at garage sales). The newer ones are made of metal. I used one they had at a Michael's crop, and it was fabulous. I don't remember which brand it was (probably Fiskars).
They are not portable, but can cut lots of paper at once, are straight, accurate, and never need sharpening. The "blade" is not sharp at all. It works by sliding one piece of metal against another. When the edges make contact, that is what cuts the paper.
Every serious scrapper should have one of those trimmers, in addition to a straight cutter and a rotary cutter.