I have to admit that I used to have both a collar and a microchip, but I've given up on the collars now. Once we got Skye, Riley and her quickly got into the habit of "accidentally" removing each other's collars while playing, then deftly bating them under a convenient bit of heavy furniture.....to this day we can't find Riley's last collar, missing in action some 6 months ago.....
However, as they are indoor cats I'm not too worried about collars. I know I could put some on them if I needed to (eg if we were taking them with us n holiday, or moving house etc) as a "just in case" measure, but it seems a bit pointless for indoor fluffies. If I ever had outdoor cats I would use both, but it would be a breakaway collar.