I have never flown on SATA, but flown numerous times on Transat. There are in fact pictures of SATA in the database, by the way.
SATA advantages are, it is roomier inside, with 8 abreast seating and I imagine an extra inch or two of seat pitch. But, they are consistently late. Air Transat offers pretty good service, they are usually on time lately, but the seating is very tight.
It's hard to pick one over the other though. Hope that helps!
"it's kind of like an Airbus, it's an engineering marvel, but there's no sense of passion" -- J. Clarkson re: Coxster