they're both bad :P if I absolutely had to pick i'd go for the 54..i'm 5'7 with a 30in inseam and the bikes i ride are 51-52. stand over height is one way of measuring size but reach is also
very important. you don't want to drop a paycheque on something that you're going to regret though - i say find something that's your size
i'm always scrounging CL and PB so if you drop us your measurements/budget im sure we can help find you something