The 3.5" does offer faster spool...... BUT in the real world that faster spool doesn't necessarily mean more performance. If you're building from scratch, there's no reason not go to 3.5". The airfilter is the most expensive part. Might as well get one with a bigger mouth. If you're wanting to get a 3.5" for faster spool and expecting large results you'll probably be dissapointed unless you really need faster spoolup time.
That being said, my 3" MAF pipe has a cross section of 6.47 square inches. An LT1 MAF has an area of 4.3 square inches. (66.46% of a 3" MAF pipe)
a 3.5" MAF has a cross section of 5.275 square inches. Even though it's a "3.5"" unit, the actual flow area is only 81.%5 of a 3" MAF pipe.
Damn. Now I know where the extra spool comes from. I need to write those numbers down.