Go for the tandem. I used trailer bikes for years and the floppy hitch creates a tail wagging dog effect that I hate. My daughter loved the tandem the first time we tried it on a rental and we bought one as soon we had the funds.
Depending on how big your daughter is, the Kid Tandem with the stoker on the front may be worth a look since she gets to see where you're going and Bike Friday 20" wheel tandems are also very kid friendly and not too pricey. Used tandems are great option if your area has a big enough tandem population. Craigslist in Portland almost always has a few for sale.