I take it you're with Be? I nearly switched to Be then in the space of 2 months they had some pretty bad problems. I'd rather stay away until they know what they're doing...
Maybe you get 'more' in terms of 'unlimited' but when it comes to sync and ping there's next to no difference. I put this down to line quality and distance from the exchange. Happily at Be 'standards' on a Zen line here
ZeN Pro
Speedtouch 585 v6