I prefer separates, better still if they are stretchy and soft. Easier to pull up/unbutton top to pump then to pull up the dress or undress altogether. Though my office has a room meant for lactating purposes, I prefer to pump at my workstation.
I only have 3 nursing tops and I wear them out during weekends only. For work, I still wear office wear. I dont't use strapless bra, so scared it'll slip off when I am engorged.