now, my computer that has TF2 on it has been broken since before the update, so i've only played with the update for maybe half an hour at my friends house, but this is what my first impressions are. brass beast slows you down too much for my liking, and IMO the extra damage isnt worth it for a class thats so slow in the first place. with a minigun you'll be killing things fast enough anyway. and natasha just got nerfed, so i'd say minigun is the best rigth now.
If you want to get the most kills/points, go with Sasha, it is stupidly good. If you want to frustrate the heck out of the enemy, go with Natascha If you want to blow out everyone on the server's eardrums, Brass Beast.