I agree with bimmer, the dct transmission is worth the price alone . If it is within you budget I would go with the IS. I did and I do not regret it one bit . There was a price spread between the models when I bought my car too , but not 5,000.00. I think I read somewhere that that is about their cost for BMW when they add the options for the IS. I might not be a 100 percent right on this , but here are some add ons you get with the IS: n 54 engine, 2 separate turbos, not one. Dct transmission, Upgraded M package . M steering wheel. Two separate exhaust tubes, metal accents on the interior, IS badging inside and out, special molding on the exterior and underbody panels. Two fog lights on the convertible. Sport button. Launch control. Paddle shifters on the steering wheel. Hk sound system.
Also, the IS is very rare and will hold its value much better.
The n54 engine was my choice , this engine has been around for a long time and they have fixed many of the early issues with it.
Get the IS if you can afford it, you will never regret it . If you buy the 335i, you will always be wondering , what if!!!!