Other things that matter:
* being part of a team, pulling towards a goal.
* stability. Contracting is awesome, but often feast or famine.
* work life balance. Yes, as a contractor you control your working hours, but it can be hard to stop working at 5pm too.
* benefits (if in the USA, health care is huge and the plans with an employer are almost always better)
* ability to explore different areas. When you are a contractor, you are expected to be top notch at what you do. Often that means you don't get to explore new things (except on your own time, which you should do). It's nice to get paid to learn as an employee.
I think that contracting is great, but so is employment.
Just like when you are a startup you should compete on your own terms (your USP), if you are looking to keep senior developers, find out what you can offer (some of the above) which contracting cannot. (How? Ask senior devs in your market!)
Source: I've been a contractor and a full time employee, about a 50% split during my career.