Queens peak is nice, rented there before. In terms of distance to MRT, QP & CT win hands down, but Stirling is also a very reasonable 2-3 min walk. That's a negligible difference to me. Stirling feels more secluded and less busy because the MRT and main road is blocked by the old Queens condo. Some people may personally not like to live along the main road because of dust, noise, traffic, etc. Regarding the "upslope" walk, I think that's unfounded. I've walked it before and the bus stop side gate of Stirling is situated at the bottom of the "hill" so it's an easy walk to the MRT.
For both QP and CT, you would definitely want the premium facing stacks if you are staying there, facing away from MRT. I personally find the MRT facing stacks too noisy / eyesore (my room at my parent's place faces main road and MRT and after so many years it still annoys me).
Between QP and CT I would choose CT - I think the facilities are nicer, seems more lux cos of the colour scheme (subjective), and has a tennis court (if that matters to you). Both QP and CT are comparatively small projects, so the facilities will be fewer and potentially less luxurious compared to Stirling (of course, you may argue that Stirling will have higher usage of facilities as it's a denser development, but in my experience that's usually not an issue as usage will die down over time). I'm not sure about the maintenance at QP and CT, but Stirling should have a reasonable maintenance fee due to the large number of units sharing costs.
In terms of valuation, as a newer and bigger condo (and arguably the "flagship" condo of the area), I think Stirling does deserve at least a 10% premium over the existing condos. With QP and CT units resale units going for around $1.8k PSF, I think that makes Stirling's $2~k PSF justifiable. At the end of the day, there are many reasons to buy a property. My point is just that I feel Stirling's valuation is still within reason.