King size pocket sprung mattress >£250 recommendation

Choosing between these two, replacing Silentnight Miracoil (was/is around same price):

1. IKEA VESTERÖY £250
ikea.com/gb/en/p/vesteroey-pocket-sprung-mattress-firm-light-blue-40470145/

2. Dormeo Wellsleep Hybrid Mattress £240
hotukdeals.com/deals/dormeo-wellsleep-hybrid-mattress-single-ps15960-double-ps19960-king-ps23960-s-king-ps27960-wcode-4277868

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can return / can't return
can try / can't try
basic foam layer / more advanced cooling foam (?)
20 cm / 23 cm
800 springs / 400 springs (guess based on 275 for double)
might be smaller / true size (?)

Thanks