I have the Proxxon one and I quite like it. Be careful though, on some of them the arm could be slightly bent on one side (I was one of the “lucky” ones, but I managed to fix it). You can made or buy different guides or circles etc.
I have a relatively inexpensive tabletop one that I got at a local craft store. It’s one that uses a hot wire to cut the foam. I do like it and have found it very useful. However, it does require some care to get square cuts.
One thing to keep in mind is that as the wire heats up to cutting temperature, it expands and will flex. This flex can impart an angle along the edge of the foam being cut. Proper tensioning of the cutting wire is important. Also important is making the cut INTO the “mouth” of the cutter and limit side-to-side cutting force.
Another tip I might offer is to first cut out a template of the shape you want from stiff cardstock and pin that to the top of the foam blank. Then use the template to guide the cut. I’ve found that this gives much more precision to the work than simply tracking the shape and then cutting along the traced lines.
Finally, don’t rush the cuts. Allow the wire time to melt through the foam without the need to force the foam into it. This mitigates the flexing of the wire and helps to keep the cuts square along the edges. Also, be a bit patient and allow the wire to heat up to its operating temp before making the cuts. Again, you can start cutting too soon and have to use some force to get the wire to cut, with just flexes it distorting the edges.
I also have a hand-held hotwire foam cutter that I’ve used on some larger (very LARGE) projects, and the same suggestions apply to those. Be aware that if you glue your foam layers together before you contour the edges, the dried glue layers can be an obstacle to the hotwire. The wire will not get hot enough to melt the dried glue, so pin the foam layers together, contour with the hotwire cutter, then glue them up.
Small diorama using foam sheets for base contours-
one thing i do with the Proxxon cutter when using the fence: i attached a 3/4" x 1/16" thick aluminum angle to the back of the table w/ 3M vhb tape. this allows attaching the factory fence both front & back of the table. i use a small “G cramp” (maybe 1 1/2" capacity).