I would. Unless you are into clanning. But 40 defence is more viable, you get rambo specced less and the majority of people you would be fighting with 30 defence would be pures or tanks. Pures would run or use overheads and dscims on tanks suck ass.
40 defence gives you barrow gloves and veng, and stuff like kingdom management and ardougne cloak 3. Most 30 defence go 40 defence for those reasons. the only reason people stay 30 def is for clanning purposes for the most part or to feel like they still are a pure in someway.
You can still get into a clan with 40 defence but it's a bit different and requires more cash in return sets etc.