Great that your GF is equally involved in the choice. But you need to be aware that if she has a Pungo and you get something like a Tsunami, what she has is the limiting factor on where you can both paddle. Handling things like on-water capsizes are much more difficult for the big open cockpit boats like the Pungo, so being safe means that you stay within the capabilities of the least capable boat in terms of managing conditions.
What you do with this as time goes on is up to you guys - but this is why I tend to recommend taking a basics class including rescues before buying boats. There is nothing like trying to handle a capsize in a Pungo for driving home why so many of us prefer those skinnier boats with smaller cockpits and bulkheads fore and aft. Honestly, I have gotten to the point where I place that in front of being able to paddle straight.
AS to your idea of creeping along the shoreline of Lake Erie - be aware that many days that will not be much of a plan because that's where the breaking waves are. Wind drives them higher as the water gets shallower... all of which means being near the shore could be the worst place. Though being outside of the breaking waves can be equally bad if you don't know how to paddle thru them without getting trashed, or are in a boat that is an invitation to being trashed in surf like the typical rec boats.
You can have fun with many things that float, from a sexy but utterly unstable racing canoe to a construction of 2 by 4 lumber and swimming noodles. But you need to be aware of where they are designed to be used and why. If you guys have two quite disparate boats, it is a consideration you will need to keep quite actively in mind.
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