Similar

Cost per minute – $.20/min
Coverage – both use the Rogers network
Technology – GSM
Unlocked phones – Yes
911 fee – $.99/mth
Both offered by Ztar Mobile
Voicemail, Call Display, Call Waiting, 3-Way Calling including for both
No activation fee, no contract, no identification req'd, no credit check
No System Access Fee
$5 startup credit
Both only work in Canada

Differences

Expiry – SO airtime expires after 365, while PetroCan only last for 180 days, unless you get $100 voucher which lasts for 365 days.
Phone Selection – although SO and PetroCan used to offer the same phones, there are now some phones which are offered by one but not the other. Example, the Nokia 2760 is offered by both, but the Nokia 2630 is only offered by PetroCan and the Nokia 5200 is only offered by SpeakOut.
SO airtime can only be purchased at 7-11, while PetroCan airtime can only be purchased at PetroCanada stations.
SpeakOut and PetroCan offer different special deals or offers
PetroCan's official website is regularly updated, while the official 7-11 SpeakOut one is pathetic and has been updated in well over a year.

Fortunately 7-11 updates their product binder in their stores a lot more often than their website.

The last time I checked it said:

Nokia 1600 $75 (candybar style)
Nokia 2760 $125 (flip)
Nokia 5200 $150 (slider)

They list other phones, but they may just be leftover old stock and/or not worth it.

The Nokia 6061 (flip phone) is hard to find and has essentially been replaced by the newer 2760 which costs the same.

The Nokia 2610 (candybar style) has a hard-to-see screen and is not worth the extra $25 over the similar (and better IMO) Nokia 1600. And the 2610 is also hard to find.

The Nokia 1112 was replaced by the Nokia 1600 a long time ago and is probably nowhere to be found. (Does 7-11 head office even know they have a website?!?!?)

PetroCanada Mobile updates their website regularly, since Starter noticed the new lower price on the Nokia 2760 which probably happened in the past few days.

I hear the Nokia 2610 is hard to find at PetroCanada as well. I'd recommend saving the $10 and getting the Nokia 1208 with twice the battery life or spending a bit more and getting one of the two higher-end phones.

So, to simplify things, each company has 3 phones in their lineup – 1 low-end, no-frills, low-cost phone + 2 higher-end but still affordable phones that have some extra features.

Some of phones offered by Petro Canada and 7-11 are the same, and others are different. For the phones that are the same, PC charges less to buy them. But if you are an extremely low cell time user (5 minutes or less per month) and like to watch every penny you spend, then you should go with 7-11. With PC you have to spend at least $40 a year for airtime, whereas with 7-11 you can get away with $25 a year.

And if you are a collector of petro-points, PC gives double points.