A smaller turbo running high boost and producing less power will be pushing fairly hot air into the motor, which makes detonation et al more likely.
I'd go for the larger turbo, less boost, more power. Part of the power gain comes from lower exhaust restriction with the bigger turbine, so don't forget that. It's probably better to push a larger volume of cooler air through the motor, ultimately more power will mean higher cylinder temps, but I'd rather be making a given figure with a bigger turbo than a small one (assuming the little one is being driven to the limit of its efficiency range).
Lag, of course, is another story