Jon,
IDFs are available from 36 - 48mm with the largest available venturi at 40mm. They are more streetable than IDAs with a better overlapping progression circuit, 3 progression holes from factory.
IDAs come out as a 48mm, but can be modified out to 51.5mm. The largest available venturi is 45mm. The are 62mm IDAs available from Gene Berg, EMPI etc.
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The progression circuit can be modified as described in other weber tuning threads.
If you have a choice for manifold, it then comes down to the style of driving you intend on.
The IDA is more cold blooded since it has no cold starting circuit unlike the IDF.