I'm looking to buy a Cervelo Dual in the next couple of weeks, but am torn between the 51 and 54 size. I'm 5'8" with a 30.5" inseam. According to bike-fitting websites I've found, I should be looking at a 51.9cm frame size. The Cervelo seat tube adjusts b/t 74 and 79 degrees, which means that the 51 size would fit me at a 75 degree angle (53.5 cm top tube length at that angle). However, I've read that the steeper the seat tube angle the better so keeping it at 79 degrees is preferred. The 54 size at 79 degrees yields a 53.0 cm top tube length.

Does anyone have any experience fitting a Cervelo that might be helpful? Should I go for a smaller frame size and adjust the seat tube accordingly? Or should I go for the larger frame size? Thanks!