The R-value of the quiet-type underlayment is nearly nil, and won't be buying ANY comfort. It may dampen some nail-squeaks if the flooring warps slightly and rises up, that's it.
The difference between 15psi foam and 25psi foam when fully supported by a slab, and with a subfloor + hardwood flooring above is mostly academic. At your very low R-values the extra R0.5 is actually substantial. XPS has the additional downside of having about 200x the lifecycle global warming potential (GWP) of polyiso, since it's blown with HFC134a at ~1400 x CO2 GWP, whereas iso is blown with pentane at ~7 x CO2 GWP. The long-term (50+ years) R-value of half-inch XPS is also about R2 rather than the rated R2.5, as the blowing agent slowly escapes over time too. The pentane in polyiso is gone within months of manufacture, and it pretty much retains it's labeled R-value. (When it's fresh off the presses half inch iso performs at about R5 or 6, but 18 months later it's down to it's labeled value, whereas with XPS it takes decades, so they are allowed to label it to a presumed "lifecycle average".)
Adding 1/4" or ~R1 under sleepers doesn't add much R, but it adds some, but it's not necessarily worth the trouble of the extra work, since it barely affects the structural rigidity.
If you can get truly flat high quality 5/8" t & g plywood it will probably work out just fine as a subfloor in this app. I've seen people get away with 7/16" OSB with other types of flooring, but I'd worry about the long-term nail/staple retention for hardwood flooring. I've read of thinner subfloors developing edge curl & waves when it wasn't t & g in this kind of stackup, but that may be less of an issue when you have the rigidity of the hardwood flooring working for you.
There are some OSB designed for subflooring that can withstand being rained on without warping (!), but most OSB will wick and retain water much more rapidly than plywood. If you have a history of flooding OSB is probably less desirable, but trenching in drains, sumps, & pumps is probably on the critical list.
Putting gasketing under sleepers adds maybe R0.1, which will not raise it's temperature substantially. The poly sheeting is sufficient to block groundwater wicking, but does nothing for air-adsborbed summertime moisture. With R3 insulation under the subfloor the slab will stagnate at only slightly above the subsoil temperature, which is in the 40s Fahrenheit for most of VT. In summertime the outdoor (and presumably indoor) dew points are in the high-50s/low 60s, and over time moisture from the air will end up in wood that is always below that dew point temp. (he dew point of 75F 60% relative humidity somewhat air conditioned summertime conditioned space air would be ~60F, WELL above the deep subsoil temps in most of VT, and even if the bottom side of the sleepers was 55F it would be taking on substantial moisture over the warmer months of the year.
Were it new construction it would be worth putting R10 of EPS (also blown with pentane, not HFCs) under the slab in a VT climate, but even half that is still pretty good. If you used 1" iso you'd be at about R6, which is a huge uptick over the bare-concrete condition. With 1" iso, 3/4" subfloor and 3/8" flooring you'd be at 2-1/8", and with a 5/8" subfloor you'd be at a dead-even 2" rise. Using 3/8" hardwood and a thicker subfloor isn't a bad way to go, since the thicker subfloor would do more for distributing the load along both axis' than 3/4" subfloor, which is really stiff along the length of the board, but totally flexy laterally from board-to-board (which is why it's preferable to run them perpendicular to joists, in timber framed floors.)