Years ago when I decided that I really was going to dedicate long term effort to photographic work on paper I went through the "which trimmer" dilemna. I finally decided on an IDEAL precision safety guillotine.
It was expensive even second hand but has now operated flawlessly for two decades with the original blade. It cuts film and paper leaving perfect edges and dead square corners with a razor sharp blade working (in pitch blackness even) with perfect safety. The spacer bar and automatic hold down clamp prevent paper or film moving around so cuts accurate to 1/1000 of an inch are repeatable. 8 ply mount board cuts like butter.
My guillotine is marked:
S/N 233603 08/1982
IDEAL-WERK D-7460 BALINGEN 1
The maximum length of cut is 360mm and I've found that adequate for all film and paper challenges. Sometimes I wish for a really big one to cut 16"x20" mount board but the "small" one is awkward enough to carry to the darkroom and back. I had the blade resharpened a few years ago as a precaution; cost $150! Now it cuts no better, no worse than before. I'm glad I bit the financial bullet in buying this thing.