Best joins the Sky Blues from Southampton on a three year deal with a fee to be decided by a tribunal.
Borrowdale,formerly with Palace, has also signed on a three year deal and his fee is subject to a tribunal.
Leon deserves great credit for making the decision to join us," said Tim Flowers to the CT.
"I had a long chat with him the other day and I was very impressed with his attitude and it is clear that he wants to come to work with Iain and progress his career, and he feels this is the best place to do that. "So we respect the way he is doing things. He is a good young player who is quick, strong and exciting to watch."
Borrowdale was at Crystal Palace during the time of Gary Borrowdale and the City boss said: "Gary has performed consistently well for Crystal Palace at this level and I think it's easy to forget just how young he is with the amount of games he has played.
"I made it clear when I joined the club that we needed strength and depth in all areas on the pitch and I believe Gary can certainly help us to achieve that."