This is a relatively simple question， but until now I have not consistently heard or seen a dominant choice. Is there a difference? And if so what is the difference.
顾客 can be explained as "a guest(客) who visits(顾) our shop," that is, a customer. Its opposite side is a retailer.
客户 (client) often refers to one or more people who comes to negotiate a business. Its opposite side is a company or a self employed person (e.g. a psychiatrist or a lawyer). I believe both words appear under Western influence.
客户 (client) is more formal, also it is used to refer to repeat or long term customers.
顾客 (customer) is more casual, most of time it is used to refer to shopper or diner, people who are not purchasing on a regular basis.
for example, if I ordered something from a store or online, I am a 顾客. But if I rent a place, a car or consistently have products being shipped to me (usually under a contract), then I am a 客户.
In other words, 客户 is someone who you have an established agreement with. 顾客 is just someone who purchase from you.
顾客 is a customer, people who buy goods or services, especially from a shop.
客户 is a client, for whom professional or organizations provides professional goods or services.
So there are mainly 2 differences between these two words：
1 客户 sounds more formal and professional compared with 顾客。
2 顾客 is more used for ordinary customers, especially for shops, and small businesses.
But there is also an obvious trend that we use 客户 more often than before to make our customers feel more comfortable and to show respect. Hope this can help.