I grew up in Freehold (Monmouth County) and now live in East Brunswick (Middlesex County). I actually do see a big difference between the two. Monmouth County is much more country. Areas like Colts Neck have large lots and life is just more relaxed with farm feels in some areas and more farm markets, delis, etc. Also, Monmouth County is very white collar as jersey suggested. Middlesex County is more North Jersey with more of a city feel. There is more traffic and a sense of overall constant busyness. I'd say if you like the country and relaxed feel, go to Monmouth. If you like the city and want to have more to do, go Middlesex.
Everyone thinks and feels differently though so hopefully we can get some more opinions. And don't get me wrong, I love East Brunswick, where I live now, but Freehold would be my first choice (East Brunswick was my husbands first choice).