If you've stayed in Marylebone on previous visits, or west London in general, I think you'd be very surprised by Hoxton/Shoreditch, whether this is a good surprise or a bad one will depend on your age and likes/dislikes. In my opinion they are exact opposites.
As previous posters have indicated, Hoxton and its sister, Shoreditch are probably the hippest neighbourhoods in London at the moment, though they're not as hot as they used to be. It's trendy, artsy, centre of the club scene and filled with young people (i.e., mainly 20-somethings). It also has its other charms -- you'll find Spitalfields market, Brick Lane, and Columbia Road market nearby. But it is gritty (for London) and safe for the most part (though native Londers may not agree). There's always people about, especially at night. Those people may or not be sober but they are out.
But in terms of tube transport, it's not very convenient to the west end. Old Street on the Northern line is the nearest tube stop and thats good if you're travelling north/south but to go west you'll need to change to another line
Liverpool street and Moorgate on the Central, Crcle, Hammersmith and City, and Metropolitan lines are about a 15 minute walk south from Hoxton. Buses would probably be your best bet. The No. 55 runs along Old Street to Oxford Circus. The No. 243 also runs along Old Street to Holborn before turing south towards Waterloo.
Here's a link that I think gives a good description:
http://www.viewlondon.co.uk/home_feat_local_shoreditch.asp