1. R9 290X crossfire configuration is generally faster because having two dedicated cards allows the gpus to run at full clock speeds without being cramped into one card. so they will be a little faster than a r9 295x2
2. R9 295X2 also has some advantages. 1, it will be a lot lower wattage due to everything being in one single card. 2, you will have more room in your PC for other cards.