A semicolon or an em dash? [duplicate]

Help! I've read everything there is on Google re: semicolon and dash use and, whilst I understand all this, I still struggle deciding when to use a semicolon over a dash to join two independent clauses. I'm an avid endasher (from UK, so endash used more than emdash), and due to their increasing prevalence in newspapers etc, I think I'm a little desensitised to them. Consider the following:

Sorry I didn't get round to helping; it's been a difficult week
Sorry I didn't get round to helping - it's been a difficult week

I couldn't go to work; I was feeling too sick
I couldn't go to work - I was feeling too sick

I ate all the food; I was starving
I ate all the food - I was starving

That's not borrowing; that's stealing
That's not borrowing - that's stealing

Now, I feel I say all of these aloud with some sort of emphasis, so I naturally hover over the emdash. So my question is... How do I decide whether my joining clause is dramatic enough to warrant a dash?

I understand dashes are more frequent in informal writing (and, if I'm being honest, I'd find myself 'stuffy' using them in emails etc) but, regardless, I'd like to at least feel I know when the semicolon is more appropriate, even if I choose not to use it!

Can anyone help? Thanks in advance!

Personally, I don't think that either a semi-colon or an en-dash is appropriate in your examples. I would use a colon or an em-dash. (I also don't think that an en-dash v. and em-dash is an American v. British issue: they have different purposes (see http://www.chicagomanualofstyle.org/qanda/data/faq/topics/HyphensEnDashesEmDashes/faq0002.html).)

A semi-colon is part-way between a comma and a full-stop: it is not for juxtapositioning two clauses in the way you have -- that requires a colon or an em-dash!

This article http://www.thepunctuationguide.com/em-dash.html has a useful summary about em-dashes v. colons and other punctuation.

I've never encountered a rule set in stone, but I understand that generally it's okay to use both. I use a semicolon when I'm adding something that's not necessarily directly related to the statement. So parts of a sentence separated by a semicolon could really just be two separate sentences. I use a dash when I'm extending and/or emphasizing the thought in some way.

Example:

I watched the car go by; I felt paralyzed with fear.

I watched the car go by - kicking up a flurry of leaves behind it.

This is probably completely arbitrary but that's how I use them. :)