There are three versions for Office365; Small Business, Enterprise and Education. I'm leaving out Office365 for Education, for it's a version for Education only.
The Enterprise edition comes in 4 flavors (E1 - E4), while there's only 1 Small Business plan (P1). However both versions are very similar, there are some differences, which can have a huge impact.
Why two versions?
Simply said, there's one version for small businessess (<25 employees and no IT dept.) and one for big(ger) companies. Hence the original names; Small Business and Enterprise.
The small business version is the "cheap" version. This "lite" version is a stripped down Office365 with most functionality and the benefit of being much cheaper. Below I've summed up most differences between the small business and enterprise edition.
Active Directory
The Small Business version will not support Active Directory Sync. If you need this functionality, you will have to choose one of the 4 Enterprise flavors.
BES support
The small business edition doesn't feature BES (Blackberry Enterprise Server).
Users
The small business version has a maximum number of users of 50. Need more? Please upgrade to the Enterprise edition.
Microsoft Office
Both versions will have the Office WebApps included. But only the Enterprise edition will also feature Office Professional Plus.
Advanced archiving
If your company needs advanced archiving features, like e-mail archiving for legal compliance, you are in need of the Enterprise edition.
Support
The small business version features full community support. If you're in need of full 24/7 phone support, you will have to get the Enterprise edition.
Indiana Jones
Yes, be like Indie and choose wisely. Ask yourself the next questions if you own a small business and want to move to the cloud. Can you really afford to lose full phone support, AD sync and advanced archiving? Or do I want to move everything to the cloud cheaply, get rid of on-premise servers and don't need Big Corp features.
Sure it's no life or death decision, but it's nice to start of the right track instead of having to upgrade after 3 months.