Since you still have your 'current' bike and are comfortable on it, I would measure it and select the size with the closest effective top tube. That will be a good starting point to fine tune the fit on the new bike.

For what it's worth, I'm your height with same inseam and longish arms. On bikes with a level top, my best size is 60cm with a top tube measurement between 57.5 - 58.5cm. But I am comfortable on most frames 59 to 62cm. Stem lengths are typically 110mm - 120mm.

How's your flexibility? If you can easily touch your toes, i would suggest a head tube length around 18cm.

edit: looking at the Merida site, i would suggest the size 56 not 54. If you can test ride even give the 59cm bike a spin.