It's hard for two reasons.
- They are different, so don't compare well.
- People have their prejudice. They have "their" platforms.
To get round the differences and ones own prejudice, I tried to focused on small comparable things, on comparable content.
The same content for each platform. Shared at the same time on each platform.
In each case the same image with similar copy. Copy changes were made only because of Twitter's character limitations. The timeframe, from left to right in the graphs, is ten months.
There are no numbers on the graphs.
For this comparison you don't need numbers to interpret the graphs. The only number I'll mention, because it helps interpretation, is that over the graphed timeframe the twitter audience was roughly four times bigger than the Google+ audience.