Right now, Public Mobile doesn't look too great with the old CDMA technology and little phone selection compared to WIND Mobile's AWS and the ability to use unlocked phones from various sources (not so many now but as WIND and T-Mobile grow, there will be more AWS phones), including the Blackberry 9700, etc.
I guess they suit their demographic well, for people that only need their phones for talking and texting and don't use smartphone applications or data, just want something simple it's good for them. But $40 doesn't include the extra $5 for voicemail and caller ID and unlimited Canadian long distance is only included for free for people that pre-register and sign up before their launch, whereas with WIND (and potentially Mobilicity when it launches), you get a wider selection of phones, coverage in Toronto, Ottawa, Calgary, Edmonton, Vancouver and more cities in the future, and for $45 unlimited talk and text with nationwide long distance and voicemail and caller ID included (plus data if you want it).
The thing Public does have going for it though is Quebec. Videotron bought out the spectrum in the auction for AWS so WIND Mobile and Mobilicity won't be coming to Quebec any time soon, but Public is launching in Montreal and plans to expand to the entire Quebec City-Windsor corridor in the long-term...so if you live in Quebec or travel there frequently, WIND and Mobilicity are out of the question.