Below are a few threads to look at regarding the differences between the 2 products. If you're doing everything with the VI client today, then you would be OK with 3i. But right now the console options are very limited as well as 3rd party support (management agents / backup software for examples).
http://communities.vmware.com/thread/136860?tstart=30
http://communities.vmware.com/thread/138546?tstart=0
http://communities.vmware.com/thread/135443?tstart=30
2. Can I add a combination of ESX3i and esx3.5 to a single VC (2.5) inventory and make use of DRS, HA features?
Yes, but you would have to use 3i Update 1 to get HA support. A few posts above will mention that HA is not suppoorted - that is not the case with Update 1. Likewise for some networking features.
3. My environment has a standard requirement for each server should have 4 NIC's (Management, Backup, 2 * production- teamed). If I use ESX3.5, then one of the NIC needs to dedicate for console which may not be assigned for any VM's. So I need to have an additional NIC for a console to avoid any security issues. Is the second console port strongly suggested? If so, I need one more NIC. Please correct and suggest me if whereever I am wrong.
With 3i you still need a management port so with your config you would still need 4 NICs with 3i. How are you doing your backup? You could consider creating one vswitch for management and backup and seperating them with VLANs if security is a concern. That way you get some redundancy for management / backup.
Where are planning to put the vmotion network and what sort of storage are you using?
