That other answer was a little cryptic, so let me help you out.
It's easy to figure out which one's faster if you do the math. Let's say you have a level 1 Foongus with an Everstone. To reach level 50, it must gain 125,000 exp. points.
To get Amoonguss to level 62, it must gain 238,328 exp. points, which is more, except you have the lucky egg, which makes it get experience at 1.5x the normal rate. So to compare the two, let's divide it:
238328 / 1.5 = 158,885
So even with the Lucky Egg, it would still take longer.
So to answer your question, it takes less time if you leave it as a Foonguss.
Oh, but there's more! You can give your Foonguss a Lucky Egg and mash the B button every time it tries to evolve. That way, it can reach level 50 at a comparative 83,333 exp. points! Of course, canceling the evolution every time it levels up can be a pain, so it's your choice.
And as a side note, going to Cafe le Wow is a ton faster than the Elite Four if you want to level stuff up fast (assuming you're playing XY).
Hope I helped!