My Two Pennies worth:
(You don`t say what your level of experience is, so writing this assuming you have recently passed your test, please excuse me if I am teaching you to suck eggs)
Sports bikes are more fun,have greater performance to grow into,are plentiful;
However, Handling can be very twitchy for the novice, insurance will be very high if it`s your first bike, parts can be expensive (esp if on Third Party insurance)
Bandits,are cheap and plentiful,Ride Magazine has a feature this month on buying a bandit.
Honda are generally the better engineered and finished of the Jap four,CBR 600 F will have enough off everything to keep you happy, whilst remaining unintimidating and easy to ride/gain bigger bike experience on.
Yamaha Thundercat 600 is similar, however finish wont stand up to UK weather as well and they are prone to rotting exhausts
Could go on for ages,suffice to say, set your budget, dont forget to look into servicing costs,get lots of insurance quotes(Mine have varied by £1500 by phoning different brokers)
Then look for whats in your price range, go and sit on a few and see if they flick your switch, because like it or not, no matter how practical you try to be, bikes evoke a personal reaction.
Lastly, have fun, once you`ve had one there is no going back, they get into your blood for ever.
Feel free to PM me if you want to discuss anything in detail
Best of luck with your quest