I've stayed many times in the area of Agia Marina (Atrion Hotel, Epimenides, Mastorakis Village etc).
Chania itself takes some beating as a beautiful town with great restaurants, stylish shopping and superb nightlife. Visit http://www.west-crete.com/crete-daily-photo.php and you'll see how fantastic western Crete is.
Must sees are:
Chania (eat at Kanario)
Falasarna Beach
Elefonissi Beach
White Mountains (inc. the Samaria Gorge)
Prevelli
West of Chania (the wild west)
German War Cemetry at Maleme
Balos lagoon (great boat trip)

I don't know Mirabello but Agia Marina has plenty of nice things. Visit the live webcam on the Mitsos restaurant website and just enjoy looking at the beach and sea.

Thanks for the tips, Richard.

We actually changed our minds from Plakias to Agia Marina as Chania sounded really nice and we do like to spend a couple of days looking round towns (Kos, Rhodes, Corfu etc).

It dioes sound like we'll have plenty to do and see, we're really looking forward to it.

Thanks again.