You have to look at what you intend to use it for. When I bought my Long Haul Trucker, I could have used a 54 or a 56. I went with the 56 because I wanted to be able to stretch out more on a tour. Both can be made to fit very easily when you are on the cusp. IT also had the added benefit of being more stable, which is very nice on long descents at high speeds. THe combination of the slightly longer effective top tube length, and 26 inch rims, also helped eliminate toe overlap. For that reson alone, I prefer the longer top tube length.

The larger frame also allowed me to be more upright without stacking tons of spacers.