Personally, I think Trello is the way to go, especially for small teams. I've used Assembla in the past, but I found that to be kind of wonky.
Generally, my advice on project management software is to pick the one that makes work feel the least like work, haha. Some PM software is pretty dry and pedantic. At least Trello *feels* a little more fun, if that makes any sense.
If you're up to the task there are some decent open source systems out there that you can try, too, like
https://www.openproject.org/. The maintenance is obviously an issue with those, but I've been keeping an eye on OpenProject for a while and it seems to be pretty slick.