Personally I ride a variety of bikes from a little on the small size to a little on the large size. I prefer a little long to a little short.
Unless it is really small, you'll likely habituate to the new bike quickly.
Personally I'm not a big fan of sloping top tubes. One either gets the seat low, and the stem awkwardly high, or the seat post way too long.
I think many racers ride bikes a size too small just to get a bit more aero position.
Your used bike, the stem is already cut, so that will limit adjustment a bit. However, it is cut fairly long, so perhaps not that limiting. Road Cyclists like their stems more or less parallel to the ground. However, that isn't a requirement. You can flip the stem over so that it points up, and perhaps gain 3 or 4 cm in height up front.
Nonetheless, if the bike is cheap (< $1000 or so), then it might be worth it for you.
On the other hand, if it is expensive (> $2000 or so), then wait and purchase what you really want, and don't spend the rest of the time you own it second guessing yourself.
Mighty long seatposts are available, at least in aluminum.