Aren't all ISPs alike? In many ways, yes. But they differ in one way that's very important if you are using the Internet for business: reliability in delivering your mail to you.
One problem all ISPs have to deal with is spam, or junk email. Some ISPs can be careless in protecting against spam, sometimes discarding your legitimate email in the process.
The worst ISPs in this respect are probably those that use the Realtime Blackhole List, or RBL, to filter out spam. The RBL is a blacklist maintained by a group of volunteers. Because the RBL is free, it is tempting for smaller ISPs that don't have the resources to filter spam in house to simply block mail from every computer on the RBL.
Unfortunately, like most vigilante groups, the RBL folks are a bit overzealous. They will put any computer on the RBL that has ever been used by a spammer, even if this computer is shared by thousands of legitimate users.
The result is, if your ISP relies on the RBL to block spam, your incoming mail will sometimes get thrown away, along with the spam. (The RBL folks warn about this in their site, but many ISPs don't read the fine print.)
There might be some Internet users who are willing to have some of their legitimate mail thrown away in order to filter out spam, but a business using the Internet must always be able to receive mail from its customers, even if some spam sometimes gets through with it.
So if you are choosing an ISP for a business account, ask whether their spam-filtering systems could ever accidentally delete your incoming mail. In particular, avoid ISPs that simply rely on the RBL to filter out spam.