The biggest gain you get from switching to sumo is the decreased range of motion. Depending on your form they won't be any easier on the back or harder on the legs than conventional. If you pull conventional with a straight back and relaly drive your hips under then you use more leg than people who do more of a straight leg dead. Conversely if you aren't really flexible you can't get into a good starting position for doing sumos and you end up still levering up with your back and not engaging your legs more. A big factor in determining whether you should do conventional or sumo is the ration of your legs. People with long femurs tend to have difficulty doing conventional pulls without levering up to clear their knees. Long femured peeps might look into sumo pulls.
Basically give them a try and see how it goes. I tried sumo for a while but am much stronger pulling conventional.
Cheers,
Scotsman