Two totally different experiences. I have been to both and would highly recommend both

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If you are looking for campy, sing-y, clappy, hokey-humour kind of fun, then Oh Canada Eh! is the one you should pick. Its the furthest away from the "tourist area", at the far end of Lundy's Lane (maybe 5-10 minutes by car from the Falls).
If you are looking for a more polished, "Vegas-style" production and are into magic and illusion, then Greg Frewin's show is the one for you. You can walk to the Greg Frewin Theatre from the Clifton Hill area. It will take you about 10 minutes or so.
Oh Canada Eh! comes with dinner included. Its served family style and is all-you-care-to-enjoy.
Greg Frewin's show comes with or without dinner. For those who select the dinner (which we did not.....so I can't comment on the quality) I know its buffet and served in advance of the actual show. So you would go early, eat, then watch the show with other guests who are attending the show-only. Drinks can be purchased by either group for enjoying during the production.
Both are excellent choices for family entertainment. At Oh Canada Eh! we were singing and laughing and had a great time. At Greg Frewin, we were mesmerized by his illusions.....left speechless at how he managed to do the things he did. I don't think you could go wrong with either.