The OP did specify engineered wood, but the likelihood of throw-rugs/area rugs being used at some point in the future, especially on an uninsulated slab with maritime zone-damp soil with a subsoil temp of ~9C (like this poster's location) is quite high, and would a mold hazard at MA type summertime dew points. He also lives in a 30-50% higher-than-US-average-cost natural-gas & electricity market, which makes the economic rationale on the space heating end more compelling than in some other places.
Insulated slabs aren't necessarily the best investment for every retrofit, but IIRC code-min for new construction is now R10 subslab in Maine (which is probably warmer than where you live.) If the goal is to lower the energy use profile every time the house is remodeled or upgraded, skipping the foam now makes upgrading it in future cost-prohibitive. The high-R home rule of thumb for this area is R-10/20/40/60 or sub-slab/foundation-wall/above-grade-whole-wall/attic-roof R values. (With air sealing and not-too-many & better grade windows houses built to that prescriptive standard would likely meet the Canadian
R-2000 standard in most of Ontario too.) Code minimums are about half that (or less), but building only to code minimum on new housing is crap, IMHO. The comfort factor of code x 1.5 and air sealing to <2ACH/50 is enough of a payback in itself for the people living there, even if there's no margin in it on resale value. (Better to spend the money on the granite counters and the 6 sidespray marble master-bath shower if the intent is to flip it in 5 years, eh? ;-) )
In a well designed home the cost of going code-min x 1.5 can be relatively cost-neutral using an iterative design process, using best-value materials & techniques to attain those whole-assembly R values, and applying the savings on the reduced cost of the smaller-simpler mechanicals to the R values. Clearly using 5" of 2lb foam cavity fill to raise the whole-wall R of a 2x6 studwall from R14 using fiber insulation to R17 isn't going to cut it in a present-value analysis of the utility savings any energy market. But adding 1-1.5" of 2lb foam to the exterior of a cellulose filled 2x6 wall for a whole wall R in the R20+ range would in most markets, as would taking it to the R25-R30 range with 3-4" of exterior EPS at ~$1/R/square meter in colder climates. The combined annual mortgage + utility payments for tight code-min x 1.5 homes in cold climates is usually lower than a code-min home, if some care was taken on how the whole-assembly R values were achieved. (Code min walls in MA are R20-2x6 or R13 2x4 + R5 exterior foam, either of which comes in at about R15-ish for a whole wall assembly. Foundation walls are required to be at least R15 if continuous foam, or R19 if open-cell in 2x6 studwall, again about R15-ish. Taking that to R20-25 isn't a huge stretch or huge expense.)
Slabs are required by code in my area to be poured over coarse stone primarily for the capillary break against ground moisture, not the paltry insulation value of the air spaces although it does add about R1-R2 if you go deep enough. But the porosity of the stone and the foundation backfill can in some cases create surprisingly significant air leaks at slab penetrations, notably sumps, which may require air-tight lids for homes to meet even IRC2012 air tightness levels.