I'm basically your height with your same inseam (5'6" with a 29" inseam). I've had this same debate with myself: size up (S) or size down (XS). You should be able to fit on both, but for me it always came down to geometry. The XS frames, to me, look super small aesthetically and aren't my cup of tea. I feel like I'm riding a child's bike. On an XS, I can really get the seatpost up and out of the frame for a more "pro" look, but generally those head tubes are short and, for me, would require too many spacers. On an S, I'd hardly need any head tube spacers, but my seatpost wouldn't be all that tall. To mitigate the whole head tube spacers vs seatpost height conundrum, I specifically looked for Endurance frames in size S that have tall head tubes, sloping top tubes, and short seat tubes. That allows me to use one 10mm spacer at the head tube and at least have 5" of seatpost showing, Not that much I know. I think this looks better than a super small frame with lots of seatpost and head tube spacers.