Secteur geometry link

The 52cm frame comes with a 90mm stem vs the 54's 100. While the 52cm TT is 11mm shorter, the reach is only 2mm shorter because of the different ST angle. Biggest change is a 17mm difference in stack, but unless you're riding the 52 with the stem already slammed then the 54 can likely be adjusted down similar to where the 52 is. That's going to come back to fietsbob's point.

What sort of rides are you interested in doing? Long tours or quick fast rides? Have you looked at the numbers generated by the competitive cyclist fit calculator? Are both bikes at the same shop?