[GlentoranMark (OP)]

Are they both 2 sides of the same coin?

I hear them mentioned constantly but they never seem to be mentioned together.

What's the difference?

[imatfaal]

Inflation was a short limited period at the very beginnings of the universe during which spacetime expanded exponentially, dark energy is a name we have given for the cause of the very slow gradual increase of the universe now. 

It is thought by some that inflation could have been caused by a vast vacuum energy and a cosmological constant many time what it is today.  Vacuum energy and the exact value of the cosmological constant are put forward as potential reasons for accelerated expansion.  So there are great connexions - but the differences (inflation lasted for a tinest fraction of a second etc, the universe was completely different) are also pretty huge as well.

To an extent - the goal of cosmology, and even modern physics as a whole, is to make everything 2 sides of the same coin.  we can use the same methods and models to explain inflation as we use to try and explain accelerated expansion

[Pmb]

I think that it's important here to note that there was a negative vacuum energy density present at the begining during the begining. The "negative" part is crucial here.