Bulging sidewalls are not like bulging muscles...
No really, the 275's on 9" rims are a wee bit much. The other alternative is 255's (as most dont make 265's). MANY people run 275's with 9" rims (like my buddies).
Although not optimal, more tire, is still more tire, and I would go with 275's over 255's.
As for 18 vs 17. Thats a price thing. 18" rims are more expensive, and so are the tires. Then you gotta get like 275-35's (not 275-40's) to come close to the overall diameter to fit in wheelwell...