Either one should be fine. You have room for another few mm of spacers if you need. You don't mention what length stem you use, but that is a factor too. If your current stem size is short, i.e. 100 mm or less, then definitely go with the smaller frame. If your current stem length is long, say 120 mm or more, then go with the longer frame. Either of those choices may force you to buy a more medium stem, like 110 mm. Those characterizations are just my opinion, but I think you get the idea. Don't just use top tube and head tube measurements, but also the reach and height that your stem adds to it. There are online calculators for this. If you are riding a 110 mm stem already, I would say either frame could work well. You may have to fine tune the fit by flipping the stem in any case. By the way I am close to 5' 8" and have always ridden a 54 cm frame in any make. My stems range from 100 mm to 120 mm depending upon top tube and seat post set back (another factor since I slam my saddle all the way back anyway).

One additional comment. You said you are buying a Pinarello online. I would just remind you that Pinarello is one brand that is often "faked" by Chinese manufacturers (aka Chinarello). Please take care that you are buying from an authentic and reputable supplier.