I need help deciding which development board would be suitable for my project.
I need to create a wearable multi-sensor device which measures temperature, humidity, carbon monoxide levels, light and movement activity, as well as it being gps enabled.
I have an arduino but it would not be practical to use it for the device I'm designing.
I guess the implied question is "Do all of you think the Lilypad will be more practical than what I have?" 
Well that depends on the device you are designing, and what you consider "practical" for it. A Lilypad can be a better choice for wearable projects (again this aspect of your device is unstated, but implied by where you made your post). However, it is going to have less space for your sketch (~14 kilobytes) and it will run at 8 kHz, half the speed of Unos/Megas/Leonardos. It would be more difficult to attach to a garment (though the right kind of enclosure could help), but the Nano would give you simiar performance to an Uno in a reduced form factor.
You need to consider the power consumption of the sensors as well, as well as how much current the IO pins can source/sync.