my first suggestion is... start with the 55 or 75. all the small equipment used on the 5.5 (e.g lighting) will be useless when u upgrade.. thus its a waste of money.
secondly, a 5.5 isnt very forgiving when it comes to water chemistry. small isnt always easier. its just cheaper. and if everything dies from a small mistake... i guess it isnt really cheaper after all.
initially, the investment for a 55/75 is larger. much larger. however, it will save u the hassle of "upgrading" later.