K.M.
I have a sit n stand. My oldest NEVER sat in a stroller until I had my son when she was 27 months. They are now 4.5 and almost 2 and they still both like sitting in the stroller. I bought mine off Craigslist to be frugal and we only use it a couple of times a month. A little more lately because we've been taking walks in the evening. It is kind of nice to have the two of them together in one place when we go some place crowded like the mall or Disney.
If your daughter likes the stroller now, you might want to get a double because she will still want to be in it when the baby comes. You can always sell it later on Craiglist or something if you find you're not getting much use out of it.
Congrats on your pregnancy!