Ugh, I run a small business (B&B) and we went with WorldPay for the first contract..useless, never again. The card machine was sent out without required accessories, it needed to be replaced twice during the 18 month contract and, in general, their customer service was appalling. Now with Barclaycard who have been much better indeed.
In summary, I’d tend to argue that the rates will be competitive from all the major companies, you will have a little room to haggle. Keep an eye out for settlement times that they offer you at the start if you don’t have fully audited accounts to prove cashflow.
That said, there are a number of smaller fish in the pond who may be able to offer more innovative services and gateways depending on your specific needs. Here it is important to understand the difference between the provision of merchant services and the payment gateway, they don’t necessarily have to be the same actor. In fact, this is where WorldPay have taken their eye off the game; they are, IIRC, the largest merchant services provider in Europe, but rely on businesses renting the card machine from them because they think they have to be linked.