Over the weekend, I've dug into a few more automated trading development platforms. There are certainly many out there. This condensed comparison is based on my own experience, online research, or reading through their documentation. I only included the ones which I find to be the best for each different level of independent developers. We'll start with the most basic and work our way up the food chain.
TradeStation
Pros: Easy to learn and implement. Large Community. Cons: Legacy system. Require work-arounds for complex functions.
Who I think it's for: Algorithmic, technical analysis (traditional indicators) trader. Light to no programming background.
Ninja Trader
Pros: Free for development use. Modern GUI. Use of major programming language, C#. Good documentation and support.
Cons: Performance on the slow side. Proprietary API. Possible stability problem.
Who I think it's for: Medium to high programming background. Low cost (free to develop or \$50/month for trading), good for independent developer that has a long development time.
OpenQuant
Pros: Good framework. Well built piece of software. Capable of advanced techniques. "Lite" version of professional quant software by QuantHouse.
Cons: Only 1 month of trial period. Documentation and support seems lacking.
Who I think it's for: Good to Advanced programming skills.
Alphacet
Pros: Latest and greatest. Codeless development capability. Faster concept to deployment time.
Cons: Cost a fortune. Not tailored for independent developer.
Who I think it's for: Really experienced ATS trader with deep pockets.