I have a Forerunner 620 HRM and the wife has a forerunner 10, so I have a slight Garmin bias (I also had a Fenix 3 but didn't get on with it).
The first decision would be whether you want heart rate monitoring and then whether you need to support other activites like cycling.
For base running functionality the mens Forerunner 10 is available for under £60 from time to time on Amazon (set up a price watch on camelcamelcamel) and Forerunner 220 HRM is being discounted to under £150 because it is shortly to be replaced by the FR225 with integrated Mio Heart Rate montioring.
Check out http://www.dcrainmaker.com for reviews and opinion.