They both are taking the other side of the trade. If the broker doesn't take the other side they act as agency to someone who does. Ultimately someone takes the other side. Why does it matter who?
A bank does not take the other side of your trade, you have to repeat your homework. They apply a very expensive commission to execute your trade. Way more expensive than a broker. Banks trade through Investment banks, they won't bother trading against you.
How about a casino? Your gambling win is their gambling loss. Thus, they have an incentive to cheat you in theory. But obviously large casinos realized that they can make even more money by playing fair and having a good reputation which attracts even more gamblers, and make bank from the house edge. Same with large brokers like Oanda. They know they will make bank as a primarily b-book broker because the math shows most traders fail. So they don't need to manipulate spreads to try and set off your step losses to win. They make more money by playing fair and having a good reputation than by cheating which is why they have such a huge client base. The small brokers don't have the large client base, so they have less of an incentive to be honest, since they need to squeeze every last penny from their tiny client base.
If you want to play Forex fairly as a day trader try the CME currency futures. If you don't have enough cash to play the Eurodollar, try the M6E. That is the fairest way to play Forex. Once you develop something robust move to one of these "fair Forex Brokers" with your cash. I would stay with Eurodollar, but that is me. Do not even touch one of these Forex prop firms deals with a stick.
By bank I meant OTC trading. They provide quote streams, respond to rfqs, and trade against you by providing liquidity.
Where is the auction when you trade bilateral with a bank? They have captive customers and provide liquidity close to what others do; after asking are you buying or selling, size, last look, ...
lol, these val accounts are side bets on sims, orders never go into the market. you think they would just give you free money to gamble?