I need to connect to WiFi and Bluetooth at the same time using an RPi 3B+. Is it possible or will one connection drop out or be very substandard while using the other?
Yes, the RPi3B+ works fine with wifi and Bluetooth together. In fact, I am currently using a Rii wifi mini-keyboard, a Logitech wifi mouse and a iRulu Bluetooth mini-keyboard on my RPi3B+, simultaneously.
Both are independent functions of your Raspberry Pi, both can work at the same time.