They both have thrust vectoring. The SM is a Russianized Su-30MKI with all the French and Israeli toys removed. A twin seater optimized for bombing. It should be inferior to the Su-35S in the AA role as it will not come with the Irbis radar, but instead will come with an older Bars derivative set. It should be a better bomb truck than the Su-35S, and cheaper too. The Su-35S wont be certified to fire every PGM in the Russian arsenal for a number of years yet while the Su-30MKI has already been certified on basically every kind of ordinance. That's really what's driving the procurement.