Whether you bottle or keg is really dependant on a number of things really.
1. Can you be bothered to bottle your beer, I find it a very tedious job so try and avoid it as much as possible.
2. Do you have somewhere you can store a keg at a reasonable temperature, in the summer warm beer out of a warm keg is no fun, this might lead you to bottling, or getting another fridge to hold the keg / kegs.
3. How long do you want to store the beer for, if you are making something to store for a long time, i.e over 6 months then bottling would be better than a plastic keg as they keep the air out much betterthan a keg does.
As to cleaning a keg.
First I would give it a good soak in a solution of hot water and Oxy cleaner powder, to actually clean it, if you can get your arm in it you can give it a good scrub.
Once it's clean you can soak it with a bleach solution to sanitise it, you will need to rinse it out with clean water a few times afterwards to remove any traces of the bleach smell.
EDIT, beaten to it by Kev, at least our responses are vaguely similar