You are in NE WI, I'm in north central MN -- probably similar climate unless you have lake effect impact. Using winter of 2010-11 as an example, since it was a cold winter, this is what I experienced. I have a sensor imbedded in the concrete about 4' in from the exterior perimeter; 2" of foam under the entire floor; 2" of foam around the perimeter and extending vertically down about 3'. Shop has R-19 in the walls, R-40 in the ceiling. Boiler and 1000 gal storage in the shop. Storage is insulated R-30 to 50.
I have the floor set at 61F, 1F differential. At outside temps above about 10F, interior air temp will be about 57-63; at outside temps down to -30F, interior air temp will be about 51-63F. The high 63F will result during a boiler wood burn, the extra heat coming from the boiler and flue in the shop. The infloor radiant makes the shop "feel" warm almost regardless of the air temp. I rough plumbed for adding a unit heater, thinking the same as you, but after two full winters (cold in 10-11 and very mild in 11-12) I never have wished I would have installed it. If I want the shop warmer than indicated, I can set the floor sensor to a higher temp. The shop always has been comfortable to work in, and actually if it was much warmer, it would be too warm for working.
The infloor tubing is glycol, the boiler and storage is plain water. I have a 5000 watt, 240v heater that I use if I am gone for an extended period, but I have found that I can be gone for about 12 days, even with below 0F weather, and the shop stays well above freezing, although in these situations I heat storage up to about 192F before taking off. During extended absences I set the electric heater at 45F. I also have a control on storage that shuts off infloor draw on storage when tank temp falls to 80F, just to guard against any possibility of the tank being subject to freezing.