The short answer is both sentences have the same meaning. I would say that usage is the only difference. Although "if it was" is regarded as incorrect, this ngram indicates the use of the more acceptable "if it were" is declining (perhaps because it is seen as formal).
The longer answer is more interesting and it tells us about the subjunctive mood in Old English.
The first question that rises is what we mean by subjunctive mood. Since we are at ELL, I will try to keep this answer simple while crossing my fingers to not start a flame war :). So... a verb can be classified by different criteria, for example:
tense (past, present, future): depending on the time when the action/event described by the verb takes place.
aspect (perfect, continuous, perfect continuous, simple): without going into details, just say, the aspect of a verb describes the action/event further (is the action/event happening before another event/action? is it a continuous action? is it a continuous action that happened before another even/action? is it none of the previous?)
mood (indicative, subjunctive, imperative)
The indicative mood is the mood used most often, because is the mood we use for actions/events that happened, are happening or will happen.
The subjunctive mood, however, is used for actions/events that didn't happen but could've, or actions/events that won't happen but could.
Here's the most important point in this answer: how can we express the subjunctive mood in English? As often happens, the answer to this question evolves with history.
Old English had more forms to express the subjunctive mood than modern English. This doesn't mean that modern English cannot express the subjunctive mood. It means that:
the forms used in Old English are in decline and are often regarded as too formal.
alternative forms are created in modern English: the use of past tenses, the use of would, should...:
some of old forms still remain:
I wish it were Summer
a Florida judge has recommended that he be allowed to work
other infinitive forms:
God save the Queen