You might be interested in these images posted by a Twitter user, showing scale versions of different real life arenas imposed on the Midtown lots (they fit):
https://twitter.com/Bertrand_SK/stat...41068477480961
https://twitter.com/Bertrand_SK/stat...84271516745728
I see the merits of the Toys 'R Us site given how it can tie into River Landing and Riversdale, but I still think the north Midtown lot is the better choice.
The BRT will be running right past it on 22nd Street, and there's an opportunity for a skywalk connection to TCU Place/Sid Buckwold Theatre (and Midtown Plaza). Then the convention centre could extend west from current TCU Place between Auditorium Ave and Idylwyld Dr. Sure, the YMCA is currently in the way, but integrate them into a redevelopment, and then the City already owns all the parking lots west of the YMCA along Idylwyld.
I also believe there's more land around the north Midtown lot that can be redeveloped so that we fully realize the much touted development spinoffs. There's the old STC depot, parking lots, and plenty of properties on Pacific Ave and Wall St ripe for something new.
I think a North Downtown location is just too far away from the core of downtown. That area is better suited being redeveloped as a mixed-use neighbourhood to supplement the downtown, as is intended. Keep the downtown action of an arena and convention centre in the downtown.