Way too early to call this race! But an interesting article about Gmail vs. Hotmail.
It's getting late (and it was a very exausting week to be doing guesstimates about a webmail service at 1AM), so I'll make this one a shortlist:
It will be as great as using Zoë.
You can compress the messages - especially if you index them for searching, as Google aims to do.
They will not try to stick it to a particular operating system or browser (even though it currently doesn't work with Safari).
They will actually earn money with the ads (more on that another day).
They will probably do it better.