They are both just partial reflections (not including the order book) of the real process that happens in exchange. If you want to answer the question yourself, it's essential to learn how Exchange's Matching Engines work. The real underlying information is what enters into the matching engine (what traders send to it). For the sake of simplicity, there are just 3 types of messages:

• send_order (buy/sell, limit_price, quantity);
• cancel_order (order_id);
• replace_order (order_id, new_price, new_quantity)

The matching engine converts this process into different type of market data updates:

• transaction (price, quantity)
• order_book_update (... various formats)

Then your data provider may extract the best bid/ask from the order book. Once you know the process, it's up to you to decide (according to the characteristics of your strategy) which information is required, how detailed, and how fast should it be.

They are both just partial reflections (not including the order book) of the real process that happens in exchange. If you want to answer the question yourself, it's essential to learn how Exchange's Matching Engines work. The real underlying information is what enters into the matching engine (what traders send to it). For the sake of simplicity, there are just 3 types of messages:

• send_order (buy/sell, limit_price, quantity);
• cancel_order (order_id);
• replace_order (order_id, new_price, new_quantity)

The matching engine converts this process into different type of market data updates:

• transaction (price, quantity)
• order_book_update (... various formats)

Then your data provider may extract the best bid/ask from the order book. Once you know the process, it's up to you to decide (according to the characteristics of your strategy and the budget) which information is required, how detailed, and how fast should it be.