Are you looking at my car? (I have a '75 with a 77 Turbo engine).
Keep in mind that the '75 is not a galvanized body, so it is more prone to rust than an SC. This might be a concern depending on where you live. 75 is a great year in CA because it's the newest 911 that is exempt from all emissions testing.
The '75 also has the cool rear pop-out windows. And it's the last year (I think) with a rear-mounted fuel pump.
Sorry, getting off track. Re: your question:
Make sure the '77 engine is really a 2.7 and not an early 3.0 SC engine by checking the type number to the right of the fan. If it turns out to be a 3.0, it's probably going to be a better engine in the long run because of its aluminum case and less tendency to pull head studs. If it's really a '77 2.7, it should be well documented, because it does have the same issues as the '75 engine.