This is a link to a very recent topic that was discussing both scissors and trimmers.
I think there are a lot of opinions on which are the best but one thing most of us agree on... pick something that can handle at least 12 x 12 paper. Most (but not all obviously) do.
Personally, I've tried a Fiskars portable version, like the grey one shown here. I loved it, but after years of use, it finally started cutting crooked lines for me and had to be replaced.
I then tried the Making Memories trimmer because of stellar reviews. I decided almost immediately that I did not care for it very much though. It did cut very straight lines (and still does). But the blade won't lift up to allow to allow for the cutting of frames (center cuts) like the Fiskars swing blade style does. I really like that feature and missed it in the rotary blade. And it is also bulkier and less portable. And to top it off, the self-sharpening, never needs replacing blade is getting dull and starting to cut jaggedly. I think that people from the topic mentioned above also note this.
I think very, very soon I am going to try Fiskars again. I think I might look into the "premium" style.
Hope you have good luck finding the right one for you. A good trimmer is a quintessential scrapbooking tools.