The track systems for side by sides and atv’s are built so they are geared down so you can run high range and still have plenty of power and ground speed. If needed you have the ability to go to low range for crawling or extreme situations. As far as the 900 goes it has TONS of torque. Also with the all new design with the 900 and all the things they did they are AWESOME! But really hard to find a used one as of now! You will not be disappointed with the 700.
2 really different machines, price and performance wise. The engines are 2 completely different designs, go with the 900 if you can but there will be a large price difference, it is a much better design. The 700 will probably be fine and if the price is right go for it and watch for a used 900 in a few years.