In reply to elliot.baker:
If you are getting a single bike to do everything (commuting and recreational riding) look at a getting a 'low-end' road bike, probably made of aluminium. Even the heaviest, entry-level road bikes will be crazy fast and substantially lighter than a hybrid or MTB. Personally I would spend a bit extra to get a decent spec group set on it, though, probably Shimano Tiagra or ideally 105.
I wouldn't leave anything made of carbon locked up on the street, ever.
Going against the grain but personally I would look at buying new rather than 2nd hand. The consensus in my club seems to be that the B'Twin Triban 540 (the drop bar version), despite being Decathlon's own brand, is actually a pretty decent road bike and very good value for money.
Buying 2nd hand is dicey. I recently got burned buying a 2nd hand TT bike from eBay, which looked great in the photos, and was claimed to be in 'excellent condition' but turned out to have hairline but structurally significant cracking to the seat tube - the seller claimed not to have noticed it. If I hadn't been really thorough checking it, I might have missed it too until it was too late. I got my money back eventually but only because I'd insisted on paying using paypal (seller had wanted me to pay via bank transfer, which was my first alarm bell). I would insist on cash on collection and take an experienced friend who knows what they are looking for, if you go down this route.
If you get really into road biking you will inevitably end up with more bikes anyway, then you will already have a bike for commuting and utility duty and/or winter riding, and can indulge your desire for carbon and electronic shifting.
> not get looked at twice by thieving thiefs... any tips?
Don't skimp on locks. Allocate a decent chunk of budget. You will never make your bike impossible to steal for a professional but you only have to make it less attractive than the other bikes locked up nearby. I use an Abus U-Mini (gold rated) and an Abus Granit Steel-o-flex (silver rated), both round the frame and one through each of the wheels. Combined they weigh a ton but I can leave the cable lock at work so only have to carry the mini D lock around. I live/work in central London, and touch wood, I've never had a bike stolen. That being said, try to avoid leaving it on the street if you can possibly avoid it, I have managed to get a bike locker at work which keeps it out of sight. If you have to lock it up, lock it somewhere really busy and public, not down a deserted side street.
Post edited at 18:21