IBM Websphere Application Server
147 websites

IBM Websphere Application Server costs money

http://www-03.ibm.com/software/products/en/appserv-was

Build, deploy and manage robust, agile and reusable SOA business applications and services of all types while reducing application infrastructure costs with IBM WebSphere Application Server.

VS

Zend Server
3,643 websites

Zend Server offers both free and paid options

http://www.zend.com/en/products/zend_server

Zend Server, the Professional PHP Distribution with Advanced Value-add Capabilities

Market Share by Top Websites

1 Zend Server is leading in Top 100K Sites, Top 1M Sites and The Entire Web.

2 IBM Websphere Application Server is leading in Top 10K Sites.

Websites Categories

1 Zend Server has better usage coverage in more websites categories. Including Science & Education, Computers Electronics & Technology, Adult, Law & Government and 20 other categories.

2 IBM Websphere Application Server hasn't got a lead over Zend Server in any websites category.

Geography

1 Zend Server is leading in most countries, including China, United States, Japan, Russia and 79 other countries.

2 IBM Websphere Application Server is leading in Barbados, El Salvador and Jamaica.