What are the specific specs of your machine? Apple's trade in partner kind of sucks and $700 seems really low if you have a fully specced out model in good condition. Did you try Mac Me An Offer? They tend to pay more and they are a much better company to deal with. I am really not even sure why Apple associates themselves with Phobio TBH unless it is the only company capable of handling their capacity.
As for if it is worth it, I would personally wait at least a few months. Let native apps for what you use come out, see how these systems hold up over a little bit of time, and let Big Sur get a little more mature. These new AS systems look incredibly promising but they are in their absolute infancy. Very little is known about them and software support has a long ways to go. Further, if you wait a few months, there is always the possibility that more AS models will launch before you buy, giving you more options to choose from.
