I've mainly shot with prime lenses, they take some getting use to fixed focus, you have to think about the shot you want, but by far the best IQs. Currently have the 300mm f4 and 300mm f2.8 (2.5kg) and both work well with canon TCs.
On the other hand, the flexibility of the zoom comes into its own.
The sigma 120-300mm OS by all accounts is much improved over the previous model that had a few issues, build quality, back focusing, auto focus performance dropping off when the light condition were favourable, however, it was still a fantastic lens, sharp and fast auto focusing and for the price couldn't be beat, but it weighs 2.5kg
Cheaper alternatives were the sigma 100-300mm f4 another good lens or the canon 100-400mm f4.5-5.6 if you can find a good copy.
Depends what budget you have to spend, and don't dismiss the used market, still some great deals to be had for your £££.