i would pay extra and get the SB700. I know you don't care about Alexa, but to me it is the future. More and more things around the house will be voice controlled. I love my SB700 Alexa because it responds even when the sound is up high. I have 2 other Alexa devices (sonos one and echo) and both don't hear me when i call it out - but with the SB700 it is amazing especially when you listen to music - believe me, it is very very convenient to have Alexa around.
The other plus side is the new Bose Music app that will control all your settings including bass, treble, and center channels. With the ST300, you will have to use the old Bose app and i think it's not easy to adjust the bass and treble or it is not possible (not sure cause i don't own one). And the new app will keep on improving as they get more and more feedbacks.
Bottom line, go with the new technology and you won't regret it in the future.