Rather than counting the number of users, I'd ask a few other questions.
Does it need to be accessed by end users 24/7? If so, you need Enterprise Edition to get online index rebuilds. If you can get maintenance windows after hours or on the weekends, then you can get by with Standard Edition, which takes indexes offline while rebuilding them.
Have you tested your IO throughput? You said you're using ESX - if it's v3.5 or earlier, or if you're not using the top license tier for v4, then you don't get multipathing capabilities. Your throughput is confined to just one host bus adapter per array. If these terms make your eyes bleed, here's the short story - you might be bottlenecked at just 200-400MB/sec, which can be saturated with a dozen SATA drives. No matter what kind of SAN you hook up, it won't matter if you can only get one HBA of throughput at a time. Standard versus Enterprise will be the least of your problems.
How do you plan to do disaster recovery? If you're thinking about using log shipping or database mirroring, then look into Enterprise's capabilities. If you're doing mirroring, then you get some slick features around corrupt page recovery by using the mirror's SAN instead of the primary.
Are you storing sensitive data or documents in SharePoint? If so, you should consider Enterprise to take advantage of Transparent Data Encryption. That encrypts the data at rest, when it's sitting on the SAN, thereby preventing someone from grabbing a SAN snapshot or a copy of your backup tapes and restoring an entire copy of your database when you're not looking.
Don't worry about compression. Enterprise adds the ability to do data compression, but don't let that sway your decision. Your SharePoint users are probably going to be storing a ton of binary documents, which don't compress very well anyway.