i'm 5'8" with about a 30" inseam, and i've heard people tell me i need everything from a 52" seat tube to a 55. this is confusing for me, especially since i am looking at the lower end bianchis, which all jump from 52" to 55" with nothing in between. so in everyone's experience, is it better to err on the size of smallness or bigness? what are the advantages/disadvantages of each? thanks in advice for your help.

p.s. i know i should just get what 'feels right' but being new to road biking, it all feels somewhat awkward to me, in all honesty