If you are using a broker offering a good demo environment (meaning realistic fills), then there shouldn't be any difference between demo and real trading, except for the psychological aspect (which could have a huge impact for manual traders).No! Not at all. It depends on the broker, but most brokers are very relaxed with filling your orders on a demo account, that doesn't happen like that in live trading. Filling at the right price is crucial to your performance.
Demo accounts should never be used to test your trading abilities, they are meant to offer you a playground to get familiar with how to execute your orders, that is all.
Never use your performance on a demo account as an indicative of your trading abilities.
If you really want to know how you are doing, trade the minimal size for six months -at least- on a live account.
IB has a good demo (or paper) trading environment. They always put your order at the end of the queue, meaning it's always the worst case scenario.
This is not my opinion but facts from extensive use of demo environment vs real trading.