From the sound of thing, either London or Paris would be the place to go for you. Tuscany is a region, so unless you're going to be specifically be in Florence, you're going to find yourself mostly in the gorgeous Tuscan countryside but not as much option for nightlife and entertainment.
Both London and Paris have a lot of option for nightlife, entertainment, shopping, music and dancing. Travelling around the city is very easy with the efficient metro/tube system running some 16-17 hours a day. So what other criteria can you use to evaluate?
Here are some factors you could take into consideration:
English speaking in London so that's a big plus for you, whereas in Paris while many speaks English they're normally French speaking, especially in social context unless you're going out with a group of people of whom majority are English speakers (which then would unlikely be locals)
2) Currency and general expenses
In London you'll be using pound sterling, which is currently high in terms of exchange rate, so expenses cost may be higher. In Paris, the currency in use is Euro, and it is also trading very strongly at the moment. In fact in recent weeks, it's nearly coming to par with pound sterling. With currently volatile market, it's hard to say how this will play out in the next few months. Buying things is slightly cheaper in Paris than London but not terribly different from one to another.
3) Concert ticket prices
I've found in the past that concert/gig tickets are much cheaper in Paris in comparison to London but I haven't been to any lately so I don't know what the situation is. However, given if the foreign exchange rate between GBP and Euro is not too big in difference, the prices are probably now comparable too.
4) Other travels
I haven't travel much of UK but I've done a good bit in France. France has an excellent rail network and you can quite easily take trips out of Paris during your off time if you wish. So if you want to go to other French cities such as Lyon, Marseille, Strasbourg, Nantes etc, you could do that very easily.
My friends have ever only brought me out to dancing in nightclubs in general in London but in Paris I've been to a few salsa clubs which are always great fun. There are tons of men there who can really dance salsa and if you love it, you can't go wrong with Paris.
These are just some pointers. I'm sure there are more that everyone else can tell you. Choose what you think you would like best. There's no point going somewhere because everyone rave about it and you wanted to be somewhere else instead. In any case, I hope you enjoy your summer abroad!
And on a final note, I prefer Paris than London but that's my preference. I have wonderful friends living in both cities but my friends in Paris get to see me more often. While in London language is definitely easier, but I do speak some French so I'm not completely lost in Paris either.
I particularly like that most buildings in central Paris are only 6-7 storeys high, with which combined with the large avenues giving me this sense of breathing space. The architecture of one building to another is often unique and to be admired too. And last but not least, the cuisine and the choices of all things delicious are just incredible.