Well, the Sentry CT won't really provide any security for your golf clubs if someone decide to slice it open. However, if someone want in, they will get in no matter if you have a hard or soft cover. Question is, how hard to you want to make it for the thief?
My opinion on hard covers that are around your budget amount:
Best warranty - Truck Gear (by LINE-X) Stealth and LXP covers, lifetime warranty.
Most durable - Probably Truck Gear LXP and Undercover Armor Flex, both are new covers, so it's rather unproven. But, both have a LINE-X costing which is probably more durable than a powder coat.
Best looking - Undercover Ultra Flex or Access LOMAX, really nice powder coats.