How long does it take from Wroclaw to Warsaw by train or bus?
Which one is cheaper?
Do I need to book a ticket in advance in Nov?
If you want to get from Wrocław to Warsaw quickly, the fastest (3h40) option is the EIP (Express Intercity Premium) train. It costs ~14EUR if you book ~2-3 weeks in advance, see http://intercity.pl for journey planner and tickets. If you're buying on the day of the journey, the price rises to ~35EUR, but you should still be able to get a seat. Slower trains ("TLK", 6-7h journey time) are cheaper than EIP if you're booking just before departure.
There is a relatively fast (4h50) coach service, Polski Bus. It will cost you ~10EUR (unless you book on the day when they release cheap tickets, similarly to cheap airlines). You have to reserve your seat, but you don't have to do it long time in advance.
It is a popular route so even if you turn up on the coach (PKS) or train (PKP) station, you should be able to get a ticket for 10-15EUR for a departure within the next two hours or so.
Rome2Rio says that you can do this by bus and/or by train. There seem to be a LuxExpress coach costing 30€ for a journey lasting 5h20, as well as a regional train costing ~10€ for a journey lasting 6h15. Other bus options exist according to the linked website, they all seem to take more than 6 hours to bring you to Warsaw.
Regarding seating reservation, I'm assuming that you'll have to book a seat on the bus/coach for sure when purchasing the ticket. On the train I would book a seat in advance since it's quite a long journey and I would want to be comfortably seated.