Definitely Unity. The forthcoming Unity 4.3 release will feature a new 2d game workflow that should make it easier to create simple games for people new to game development.
Maybe you could still do an RTS, but cut it down to the bare bones of what it needs to have.
Look at a game like Rymdkapsel. It's an RTS, but it cuts a ton of complexity out of the gameplay. By doing that, and focusing on some simple but fun mechanics, and a very simple art style, you've got an RTS that a small team or a lone programmer could implement.
Rymdkapsel cost $4 and they've got at least 10,000 downloads on Android and who knows on iOS so that means they've probably cleared north of $50k to maybe over $100k.
Absolutely, that's the exact genre I'd want to target. The RTS world is way too baroque, I love the minimalists.
The problem is that even a simple RTS has some brutal challenges - pathfinding is hard, and if you want to go multiplayer you have to find a way to get lockstep logic (in C#/Unity this means doing all your simulation math using custom fixed-point numbers and using lookup arrays for all your trigonometry) - that and you need unit-AI.
