I just bought my King Koil mattress from Robinsons at Centerpoint.
I tried mattress from Simmons, Sealy, Serta, Slumberland, Dunlopillo, King Koil & unkown brands before deciding. Tried at small shops as well as galleries and Robinsons. Overall, I feel the Robinsons one is still the most comfy. Even the King Koil ones at different places have differences. Some subtle but the feeling after lying there for 15 mins or more is a bit different.
I tried the mattress that I eventually bought about 4-5 times: weekday lunch time, weekend (start of shopping trip), weekend (end of shopping trip), weekend (mid of shopping trip).
My advice: Try all different kinds and at different types of places (small shop, gallery, departmental stores). Dun listen too much to what salesmen say. Trust your own feeling. If try one day not enuff, try a few times.