The architecture is different.
The Q9400 has R0 stepping.
The i5 750 has B1 stepping.
Core 2 Quad models comprise two dies, each containing two
cores.
Core i5 750 has 4 cores all in a single die, and has the
northbridge built into the die.
Regarding performance, i recommend you to do a google search of
the relevant benchmarks.
Stepping across different architectures is irrelevant. Just
stating different architectures is enough.
AMD vs Intel, AMD wins price-performance, and future-proofness.
Intel wins performance. Intel isn't very future-proof unless you go
for LGA1366.
