That's a long question ! ;)
I'll try to summarize
1. Secure environment V.S secured apps (containers)
Most (all, AFAIK) alternative offers will offer to create containers to execute an app securely. Balance offers an environment container, where business apps and data are stored all together but, for features like hub, you can merge personnal and work data in a single environment; you only have to provide your work password to interact with. These environment containers are the single waterproof solution to prevent data to be shared, corrupted or leaked between work and personal.
Of course, it also prevent problems of compromised OS (say, key logger, for instance).
2. OS centric security
It's not a mystery: Windows platform is a virus/trojan nest. Problem is that if you have a combo like desktop+tablet+phone and have granted rights for these environments to interact, then your exposition to malware is greatly increased. Plugged to an infected computer, your smartphone is vulnerable.
3. TCO (total cost of ownership) & integration
BES offers a multi-platform integrated environment, as of date the only able to manage BB10 devices (besides Fixmo, where there's a license agreement for DOD) and iOs and Android devices. Competitors often integrates several layers to offer a global (without BB) offer. This multi-layer architecture requires more time, more efforts for a seamless (and costless) integration.
That's what I remind ... but I'm no BES salesman ! ;) :D
ah ! by the time (tongue out) I wrote this, I got this in my mailbox. Really worse a read !
How BlackBerry Balance works Potential BYOD legal issues you may not have thought of