Should I choose 26" or 28" size BTWIN Triban 100 with dropped handlebar? My height is 5 ft 9.5" (176.5 cm). I went to Decathlon and tried out both the 26" and 28" bicycles there. Both sizes felt comfortable. But I cannot really tell unless I ride for a long distance. I had to raise the seat of the 26" and had to very slightly decrease the seat height of the 28".

Which bicycle size should I choose?
Vertical height is easier to adjust than 'reach' or the horizontal distance from the handle bar. Check which one is more comfortable there. I would err on the smaller size myself, as long as it isn't too small, since the reach being even slightly longer causes neck and other pains.