I've been using Harvestapp for over 5 years now. Still really enjoy using it. I feel like they always make well-thought decisions in developing features and making design changes. The tool itself feels really solid and is easy to use. My advice would be to keep things simple at first and focus on tracking your time and invoicing in a simple tool like Harvestapp. As for project management; in my experience, there is no tool out there that fits all your project management needs combined with invoicing. I keep that separate.