Do you actually mean a guillotine or a photo/paper trimmer?
I would recommend a Dahle photo/paper trimmers as they are of excellent quality - a wee bit pricy on some models - they are of the rotary trimmer style - which means the blade self-sharpens. These last a long time - my current one I use a lot and have had it for around 7 years no probs.
However, I also use a craft guillotine [X-cut] which the blade is actually in the arm as you bring it down, it cuts. Being a papercrafter, it cuts through heavy cardstock, paper and chipboard/mountboard like butter .......
Both start around £20 upwards depending on size required.
HTH
Hels