Oddly, Vodafone and 3 aren't offering the same plans: there's some clear differences (though both roll over unused data if you top up before it expires). Here's the Vodafone options:
The $14.95 for 1GB is cheaper than rival offers from Telstra and Optus with a similar expiry date. If you were on the road for a month and using the iPad as your main device, the $49.95 "unlimited" option might appeal. Vodafone doesn't appear to have a month-by-month option:
Here's what 3 will be offering prepaid — it uses a confusing scheme where you get 'bonus data' every time you recharge, which makes it look like a slightly better deal than Vodafone, though this can take up to 48 hours to show up:
If you want a month-by-month plan, you can pay $15 for 1GB, or $29 for 3GB. Overall, these plans aren't radically different to the existing Telstra and Optus choices (save that they won't go on sale until the 28th — both the Telstra and Optus options are shipping already). As ever, network coverage might be your most important factor.