I've had both the OP6 and 21U turbine housings on my R33, look up my results in this thread and make a decision.
If you're used to full boost by 3.5k, going for the 21U version is a no-brainer, especially for a street car. 500rpm difference in response doesn't sound like much, but it's a big difference.
The 21U should be well and truly on full boost by 3,500rpm, mine hits it by 3,400 and is running PULP.