GUI
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Vmware server only provide web gui, so the only way to access the gui is via a browser..... terribly slow to operate... infact the slowest gui that i have seen anywhere.... but has the advantage that you can go through firewalls since most offices allow http port access... you can directly see the display of the vm through http, no extra work required.... but has the annoying habit of asking for installation of the same plugin when different vmware server hosts are accessed
virtual box does not have a good web-gui... but most of the times you can ssh into the machine and launch the GUI from command line and should be able to the see the GUI through X11 forwarding... the GUI is reasonably good and intutive and definitely tons of times faster than vmware server web-gui... but has the problem you need to use SSH X11 forwarding if you are accessing a remote host
Unfortunately GUI does not allow to start a machine in background... to start a machine in background you have to
VBoxManage startvm -type vrdp ... this might be irritating sometimes while using GUI
Unlike vmware server which allows certain VM attributes to be modified while the VM is running, vritualbox would completely disable any modifications to a VM if it is running... This might be incovenient if you are doing some testing like, network, where you want connect a certain device to a different network
Display of VM
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vmware server only allows see the display of the VM from web-gui.... after installing a plugin
virtual box will allow to see the display if you are running directly from GUI as a foreground... otherwise use rdesktop to connect to a certain port
General
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vmware-server would not allow to set the order of boot from GUI.... virutal box does
Network
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Both provide NAT/Bridge/Host-only-network , or equivalent
With vmware-server you would have to create the network you wish to use, if you wish to use a bridge with eth0, you would have to create that bridge using vmware-config.pl which would not allow the operation if there are existing VMs running...
virtual box allows you to directly the host device to which to connect from GUI, without and additional configuration.... host-only networking or private networks between virtual machines, can be created on the fly by just specifying a name for the internal network and any machines connecting to internal network with that name should be able to talk to each other
Overall networking of virtual box far outperforms that of vmware-server
Performance
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Overall the performance of virtual box seems to be better than that of vmware-server... but we are only to publish any numbers for vmware-server without certification from vmware
Personally I'm waiting for IBM to jump on the band wagon and sue both TomTom and Microsoft for patent infringement for using electronic devices in implementing data reading. Then Oracle decides they want a slice of the pie and sues everyone for infringing on their patent to use patents to sue other companies. And just when this mexican standoff starts to get hairy, we get HP joining in claiming that everyone has violated their patent of patenting patents. At this point all the big guns come out and the pantent suites start flying until you need a a meter by meter poster to diagram out the connection using a 10pt font. The Lawyers then go to work, bankrupting all the companies and then turn around and ask the US government for a bailout. Which is when AIG strikes claiming that it would violate their patent of receiving government bailouts.
At this point the rest of the world decides to nuke the US to stop the disease from spreading.