Ho hum.
I think it's half a dozen of one and six of the other.
Blairgowrie does not have a railway station and is a one hour bus ride from either Perth or Dundee. The field the games are held at is a short walk from a bus stop- not right outside.
Probably local lodging in Blairgowrie is already taken.
Braemar is a two hour bus ride from Aberdeen- rather less frequent buses than the ones to Blairgowrie.
If you are going to have a car anyway then it would make it easier for either games.
Accommodation wise with Perth and Dundee within the hour Blairgowrie is probably easier, if everything is booked in the twin villages.
Braemar is the games the Royals go to. Blairgowrie more of a local one.
Any games will be a memorable occasion, and have more to see and experience than you can fit into the day.
If I was you I would play around with accommodation in Blairgowrie, Perth and Dundee on booking.com and with Premier Inn or other Chain hotels (likewise for Aberdeen)- but book direct if possible.
If everything else is equal, within your overall itinerary, then toss a coin.