Bittrex is one of the leading cryptocurrency exchanges by 24-hour trade volume. While the platform's liquidity and variety of trading pairs are best-in-class, the user experience leaves a lot to be desired. If you're looking for a specific altcoin, Bittrex might be a good match, otherwise, I'd keep looking.
Coinbase is a stable, well-designed platform for buying and selling cryptocurrency. The founders have intentionally chosen stability over feature bloat. Long waiting periods for funding, few supported currencies and premium fees might deter some users but the overall experience is perfect for beginners.