@Tal,
In terms of performance an unmanaged VPS will be better than a managed one. A Managed VPS will almost always come with CPanel which is great for shared hosting and for hosting companies to easily manage your VPS for you.
You can still install any of the Opcode caching tools (APC, eAccelerator, XCache, MemCached) on a managed VPS with CPanel but you would have problems getting Nginx to work.
Performance Issues with a CPanel Managed VPS
- Un needed services and processes (TailWatch, entropychat, exim, imap, ipaliasis, named, spamd, syslogd, etc…)
- The Cpanel UI uses up valuable resources
- Your stuck with Apache as the only server option
- Only options for running PHP are dso, cgi, or fcgid (no mod_fastcgi)
Issues with Managing your own VPS
- Security (Your will be responsible for the security of the VPS
- Learning Curve
- No support to fix something you screw up or overlook
- Mail servers and FTP servers are difficult to get working
- You will be doing everything via command line/SSH
Resources to Help You
If you do decide to go the unmanaged route there are a lot of great resources to guide you through every step of setting up your VPS
-
VPSBible (membership site)
The Bottom Line
With unmanaged, you get a blank partition on a hard drive, a power cord and a network cable, maybe a wiki, some docs a forum, and Google. If your comfortable with this then go for it. You will learn so much by setting up your own server environment.