Mid-cap shares are those companies who have been in the market for quite some time and have a potential to become a large-cap in future.
Compared to the small-cap companies, mid-caps have proved their performance in the market.
On the other hand, small-caps are those companies who have a small market capitalization and usually includes the start-ups or companies in their early stage of development. They haven't proved their worth in the market and hence, they can be a potentially big gainer as they are yet to be discovered.
However, the risk involved in these companies are also way too high as the failure rates of the small caps are large.
If you are new to stock market and are not seasoned to evaluate companies with expertise, then it makes more sense to invest in mid-caps. These companies have a lower risk compared to small-caps and are capable of giving high returns to their investors.
I hope it helps. Happy Investing.