I bought a Simplisafe system about 6 weeks ago, and at the time they were running a 50% off sale. The base unit and keypad are relatively cheap, and really easy to install.
One consideration for using wireless is that sensors are relatively expensive ($20-40?), and if you add up all your windows and doors, and then throw in a water leak sensor and a couple temp sensors, you begin to have a much more expensive system. On the other hand, wired systems are a real PIA to install (and run wiring) if the house is already finished, but the sensors themselves are usually just a cheap pair of magnets.
I use to have an old Honeywell system that also offered hybrid sensors. The sensor was wireless, but also had multiple connections for wired sensors. You could use one wireless sensor to cover multiple windows if they were close enough make the connections.